Ahlul Bayt (Ahl al-Bayt) is a phrase meaning People of the House, or family. In the Islamic tradition it refers to the household of Prophet Muhammad (pbuh).
Ahlul Bayt (Ahle Bait) in simple terms is to put trust of guidance to Aal-e-Muhammad [descendents of Prophet Muhammad (pbuh)], through Fatima Zahra (pbuh) and Imam Ali (pbuh) and their descendents. Bayt is to give the Oath of Allegiance to Imam Ali (pbuh) and accept his guidance.
Ahlul Bayt (Ahl al-Bayt) or household members of Prophet Muhammad (pbuh) refers to his daughter Fatima Zahra (pbuh), his successor and son-in-law, Imam Ali (pbuh), their two sons Imam Hasan (pbuh) and Imam Hussain (pbuh), and the nine Imams from the lineage of Imam Hussain (pbuh).
Ahlul Bayt (Ahl-e-Bait) in Noble Qur'an
The Purified Ones: The Highest degree of purity means to be kept constantly away from all the causes of impurity. This is termed as the state of infallibility in knowledge, character and action. It could have been applied generally to the whole mankind who are keeping aloof from all the impurities as the word of Allah (SWT) commands; but, Allah (SWT) expressively has confined His order to certain group of individuals by excluding the rest of the mankind from it in His divine will by declaring Ahlul Bayt (Ahl al-Bayt) as the persons purified by Him to be constantly in touch with the Noble Qur'an in its original, hidden, well protected, exalted and purified form. It was Allah's (SWT) wish to remove all blemishes from them, as mentioned in the Verse of Purity (Ayat Al-Tathir) in the Noble Qur'an:
The Verse of Purity (Ayat Al-Tathir): "... Allah only desires to keep away the uncleanness from you, O people of the House! And to purify you a (thorough) purifying." Noble Qur'an (33:33)
The above verse from Noble Qur'an, is known as Ayat Al-Tathir, refers to the members of the household of Prophet Muhammad (pbuh), which include exclusively Prophet Muhammad, Imam Ali, Fatima Zahra, Imam Hasan, and Imam Hussain (peace be upon them all). Hence Prophet Muhammad's (pbuh) other offspring, wives, sons of paternal uncles, and dwellers of his house are not to be called as Prophet Muhammad's Ahlul Bayt (Ahl al-Bayt).
The argument is based on the genuine and authentic traditions narrated by Sahaba, companions of Prophet Muhammad (pbuh) recorded in both the Sunni and Shia sources.
Ahlul Bayt (Ahle Bait) in Hadith's
1. Hadith-e-Kisa (Hadith of the Cloak):
It is narrated from Umm al-Momineen, Umm Salamah that once Prophet Muhammad (pbuh) was in her house lying on a mattress, covered with a cloak from Khaibar when his beloved daughter Fatima Zahra (pbuh) entered with a dish called al-Khazira (a kind of food). Prophet Muhammad (pbuh) asked her to call her husband, Imam Ali and her two sons, Imam Hasan and Imam Hussain. Fatima Zahra (pbuh) called them and as they all sat together to eat, Allah (SWT) revealed the following verse of purity (Ayat Al-Tathir) to Prophet Muhammad (pbuh). "... Allah only desires to keep away the uncleanness from you, O people of the House! And to purify you a (thorough) purifying." Noble Qur'an (33:33)
Upon this Prophet Muhammad (pbuh) covered them all with his cloak and lifting his hands towards the sky said: "O Allah (SWT)! This is my family and the nearest of my kin, keep away from them uncleanness and keep them pure as pure can be." Umm al-Momineen, Umm Salamah adds that thrice Prophet Muhammad (pbuh) repeated these words and when she poked her head under the cloak and asked him, Am I with you? In a refraining gesture, Prophet Muhammad (pbuh) said twice: "You are (also) among the righteous."
Thus it is clear from above discussion that Prophet Muhammad's (pbuh) daily habit of stopping at his daughter Fatima Zahra's (pbuh) house and addressing her household as Ahlul Bayt (Ahl al-Bayt), was not without reason. In fact he was expounding the meaning of the term Ahlul Bayt (Ahl al-Bayt) and practically explaining to the Muslims the verse of purity (Ayat Al-Tathir) and the particular persons meant by it. To be more precise, he was drawing the attention of the Muslim nation towards the significance of his Ahlul Bayt (Ahl al-Bayt) and their leadership after him so that the Muslims should love, obey and follow them.
2. Mubahala Tradition or Incident of Mubahila (Imprecation):
In the south of Arabia, there is a place called Najran. There lived the Christian tribe of Najran who staunchly believed that Prophet Isa (pbuh) i.e., Jesus was the son of God. Prophet Muhammad (pbuh) invited them to accept Islam. In response to that, a delegation of their priests and elders came to Madina. They wanted to discuss religion with Prophet Muhammad (pbuh).
Their argument was that Jesus was born without a father through Virgin Mary, so he was the son of God. Prophet Muhammad (pbuh) explained to them that just as Allah (SWT) created Prophet Adam (pbuh) without a father or even a mother, in the same way He created Prophet Jesus (pbuh) without a father.
Hence Prophet Jesus (pbuh) was a servant of Allah (SWT) as was Prophet Adam (pbuh). The Christians did not agree with this simple truth. Christians stuck to their belief that Jesus was the son of God. Prophet Muhammad (pbuh) asked them to wait. Then came the following Revelation (message from Allah (SWT)):
But whoever disputes with you in this matter after what has come to you of knowledge, then say: Come let us call our sons and your sons and our women and your women and our souls and your souls, then let us be earnest in prayer, and pray for the curse of Allah on the liars. Noble Qur'an (3:61)
It was then agreed that the Christians seek Mubahila (Imprecation) with Prophet Muhammad (pbuh). The meaning of Mubahala is that both of them invoke from Allah (SWT) that he, who speaks the truth, may survive and he, who is not speaking the truth may perish.
On the day of Mubahala, Prophet Muhammad (pbuh) took with him his grandsons - Imam Hasan and Imam Hussain, in place of his sons. In place of women, he took his daughter Fatima Zahra. In place of his souls, he took his cousin and son-in-law Imam Ali. But on seeing the radiant faces of these sinless members of the household of Prophet Muhammad (pbuh), the Christians got scared. They decided not to seek Mubahala any more but accepted their defeat. They agreed to pay tribute and returned home.
This is how Prophet Muhammad (pbuh) showed to the world, who the true and real members of his Ahlul Bayt (Ahl-e-Bait) were.
3. Mawaddat al-Qurba (al-Mawaddah Fil-Qurba) or Expressed love for the close of kin:
In this Last Word of Allah (Noble Qur'an), Prophet Muhammad (pbuh) is being commanded to ask the believers to love his kith and kin (that is his Ahlul Bayt (Ahl al-Bayt)) and that would be the return of his apostleship. Hence, to Ahlul Bayt (Ahl al-Bayt) has been obligatory (wajib) command of Allah on every Muslim.
Say (O Muhammad, unto mankind): I do not ask of you any reward for it but love for my near relatives. Noble Qur'an (42:23)
Hazrat Abdullah Ibn Masood (RA) narrated: One day, we were accompanying the Prophet Muhammad (pbuh) in one of his journeys when a Bedouin with an orotund voice shouted at us, "Muhammad!"
"What do you want?" answered the Prophet Muhammad (pbuh).
"What is it if an individual loves a people but he does not imitate them in deeds?" asked the Bedouin.
"One will be attached to the one he loves," replied the Prophet Muhammad (pbuh).
"Muhammad," shouted the Bedouin, "Call me to Islam."
The Prophet Muhammad (pbuh) said: "You should declare that there is no god but Allah (SWT) and that I am the Messenger of Allah (SWT), offer the prayer (Salat), defray the zakat, fast during the month of Ramadan, and perform the hajj to the Holy House."
"Muhammad," asked the Bedouin, "Do you ask for wage for so?" "No," replied the Prophet Muhammad (pbuh), "I do not take any wage except that you must regard the relatives." "Whose relatives? Mine or yours?" asked the Bedouin. "It is my relatives," answered the Prophet Muhammad (pbuh). The Bedouin said: "Give me your hand so that I will declare allegiance to you. No good is expected from him who loves you, but not your relatives."(Bihar ul-Anwar)
4. Hadith al-Safinah or Safinah Tradition:
Prophet Muhammad (pbuh) has compared his Ahlul Bayt (Ahl al-Bayt) to Noah's ark. Whoever loves and follows them will attain salvation and whoever violates their sanctity will drown. While holding the door of Holy Kaaba, Abu Dharr told that he had heard Prophet Muhammad (pbuh) say, "My family among you is like Noah's ark. He who sails in it will be safe, but he who holds back from it will perish."
In another place Prophet Muhammad (pbuh) said: "The parable of my Ahlul Bayt (Ahl al-Bayt) is that of the boat of Noah, whoever gets aboard it is saved and whoever stays away from it is drowned."
5. Hadith al-Thaqalayn or Saqlain Tradition (Hadith of the two weighty things):
Prophet Muhammad - may Allah (SWT) bestow peace and benedictions upon him and his pure Progeny - said: "Verily, I am leaving behind two precious things (thaqalayn) among you: the Book of Allah and my kindred (itrah), my household Ahlul Bayt (Ahl al-Bayt), for indeed, the two will never separate until they come back to me by the Pond (of alKawthar on the Day of Judgement)." [Hadith-e-Saqlain]
Since the Ahlul Bayt (Ahl al-Bayt) carry as much weight in the eyes of Allah (SWT) as the Noble Qur'an, the former has the same qualities as the latter. Just as the Noble Qur'an is true from beginning to end without the shadow of untruth in it, and just as it is incumbent (duty) of every Muslim to obey its commands, so also must the Ahlul Bayt (Ahl al-Bayt) be perfectly true and sincere guides whose commands must be followed by all. Therefore there can be no escape of accepting their leadership and following their creed and faith. The Muslims are bound by the Hadith of Prophet Muhammad (pbuh) to follow them and no one else.
Imam al-Shafi'i, one of the four Imams of the Sunni school of thought, in his famous verses says, "Let everybody know that if the love of Prophet Muhammad's descendants means to be a Rafizi, I am a Rafizi."
Imam al-Shafi'i also says, "O Ahlul Bayt (Ahl al-Bayt)! Allah (SWT) has made it obligatory in the Noble Qur'an to love you. It is a matter of pride for you that without invoking blessing on you, prayer is not valid." Again he says in his verses, "After having seen that the people have chosen different ways which have led them to the Ocean of deviation and ignorance. I have, in the name of Allah (SWT), embarked the ship which may lead me to safety. The Ahlul Bayt of the Prophet Muhammad (pbuh) are that very ship, we have been ordered to hold fast the rope of Allah (SWT), and that rope is their love."
Conclusion: Prophet Muhammad (pbuh) and his Ahlul Bayt (Ahl al-Bayt) sacrificed all the worldly comforts and even their lives in order to convey the true religion of Islam to us. In return of their sacrifices, we are commanded by Allah (SWT) to love them. The aim is that through their love, we follow their footsteps. We follow the true teachings of Islam taught by them, so that we live a good and decent life in this world and earn the pleasure of Allah (SWT) in the life hereafter.