One day Prophet Musa / Moses (pbuh) was passing through a barren hill. He noticed a pious person busy in prayer sitting in a cave of the hill. Prophet Musa / Moses (pbuh) felt the urge to meet and talk to the pious person. Prophet Musa / Moses (pbuh) went near and greeted him. The pious person asked disinterestedly, "Who are you?" Prophet Musa / Moses (pbuh) said, "I am Musa!"
The pious person asked, "Are you the Prophet Musa / Moses (pbuh)?"
Prophet Musa / Moses (pbuh) said, "Yes!"
The pious person said, "Then ask Allah (SWT) to Grant my wish!"
Prophet Musa / Moses (pbuh) asked, "What is it that you want?"
The pious person said, "Since the last one hundred years I am sitting here praying to Allah (SWT). I don't do anything other than the Prayer. Ask Allah (SWT), what He will give me for my efforts?"
Prophet Musa / Moses (pbuh) said, "I shall find about it immediately." He immediately climbed up the hill and called aloud, "O Sustainer Allah! This person wants to know about the reward for his prayers! Tell me, what shall I inform him?"
He heard the sound, "O Musa! Tell him, we shall inform him about his reward tomorrow."
Prophet Musa / Moses (pbuh) went to the pious person and conveyed the message to him. The pious person said, "Its fine! Tomorrow is not much of a time!"
The pious person was in the habit of going to a nearby canal early every morning for bathing and fetching some water for his use. Therefore, the next morning too he started for the canal but he lost his way and reached somewhere else. Because of the sun's heat he became very thirsty and tired. He sat on a lonely boulder and started thinking about his death. At that time he found another person coming from the other direction. He waved to the person to come to him. When the person came nigh, he asked him to help him with some water.
The visiting person said, "Where is any water in this wilderness? Whatever little I have is only for me."
The pious person started crying. The other person said, "Fine! What will you give me if I give you a tumbler of water?"
The pious person said, "I don't have anything. I have been busy in the cave praying for the last one hundred years."
The other person said, "If you agree to transfer to me the Rewards for your prayers of one hundred years, I shall give you a tumbler of water."
The pious person thought, if he lived, he could pray for a further period to get the Reward from Allah (SWT). He said, "I am ready to transfer to you my reward for the hundred years' of my prayers.
The other person gave him a tumbler full of water and the pious person returned to his cave.
In the morning Prophet Musa / Moses (pbuh) got a revelation that the pious person transferred his hundred years of prayer to another person in exchange for a tumbler of water. Ask the man to settle the value of the tumblers of water that he had been drinking during the last hundred years!
Therefore, Prophet Musa / Moses (pbuh) went to the cave and told to the person that he had a message for him from Allah (SWT).
The pious person said, "O Prophet Musa / Moses (pbuh)! I have already sold my hundred years of penance!"
Prophet Musa / Moses (pbuh) said, "Yes! I know. But Allah (SWT) has said that when the value of one hundred years of prayer is one tumbler of water, then he must settle the value of the water he had been drinking over the last hundred years."
When the pious person heard this, he was shocked and shouted, "O Prophet Musa / Moses (pbuh)! Recommend to Allah (SWT) to forgive my sins! Allah (SWT) is Beneficent and Merciful!"
Prophet Musa / Moses (pbuh) got the Revelation, "tell the pious person! Your penitence of this moment pleased us more than your prayers of a hundred years and we have given to you rewards for a thousand years of prayers!"
Which then of the bounties of your Lord will you deny? Noble Qur'an (55:13)
Imam Ali (as) says: "The sin that makes you sad and repentant is more liked by Allah (SWT) than the good deed which turns you arrogant."