The most beautiful and valuable hour to Imam Musa, peace be on him, was that when he was alone with Allah, may His name be exalted. He devoted himself to Him with all his feelings and sentiments. The narrators have mentioned that when he stood before Allah, the Most High, to perform the ritual prayers or to say a whispered prayer or to supplicate, he wept so much, his heart beat and became disordered out of fear of Allah. He spent most his time in performing prayers; he used to pray supererogatory prayers throughout the night so that he would make them extend until the morning-prayer, then continue them until the sun rose. He would remain prostrating himself before Allah without raising his head from prayer and praising Allah until the sun came near to descending (from its midday zenith).One of the aspects of his obedience to Allah is that he entered the Mosque of the Prophet, may Allah bless him and his family, at the beginning of the night and prostrated himself before Allah for a long time and said sincere words out of fear of Him: "The sin is great with You! Therefore let forgiveness seem good to You, O Worthy to be fear and Worthy to forgive!" He repeated these words with turning to Allah in repentance, submission, and weeping till morning.
When the tyrant of his time, king Harun al-Rashid, threw him into the dark cells of prisons, he devoted himself to obedience and worship to the extent that he with that dazzled and perplexed the intellects. He thanked Allah for giving him free time to worship Him, saying: "O Allah, you know that I used to ask You to give me free time to worship You. O Allah, You have done that. To You be praise."
The Imam set a record in worshipping, for none was like him in obeying Allah and devoting himself to Him; his soul adored Allah and deep faith in Him took root in his heart.
Al-Shaybani has talked about Imam Musa's too much worship, saying:
"In some tens of years Abu al-Hasan Musa, peace be on him, would perform a sajjda after the whiteness of the sun until its coming near to descending (from its midday zenith). His opponent Harun al-Rashid acknowledged that he was the ideal of turning to Allah in repentance and faith; that was when he kept him in the prison of al-Rabi'. He looked from the top of the palace and saw a garment thrown in a special place of the house and did not change its place. He would be astonished at that and ask al-Rabi': 'What is that the garment I see at that place every day?'
"Commander of the faithful, that is not a garment; that is Musa b. Ja'far; every day he performs a sajjda after the sunrise to its coming near to descending (from its midday zenith)," replied al-Rabi'.
So Harun was dazzled and showed his admiration toward Imam Musa, saying: "Surely he is among the Hashimite monks."
Having heard Harun acknowledging the Imam's asceticism and renouncing the world, al-Rabi' turned to him and asked him to release the Imam and not to harass him, saying: "Commander of the faithful, why have you harassed him in prison."
Harun replied him with an answer void of mercy and piety, saying: "How far! There is no escape from that!"
Surely Harun knew the achievements and asceticism of the Imam, but he was blinded by his craving after the world and his loving the kingdom, and so he exerted pressure on him. We will fully explain that when we speak about the heavy misfortunes and black ordeals the Imam met from Harun.
When the Imam was imprisoned in her brother's house, al-Sind b. Shahik's sister has narrated the following on his worship, saying: "Surely when he had performed the evening prayer, he praised Allah, gloried Him, until midnight. Then he rose and performed prayers until morning, so he performed the morning prayer. Then he praised Allah until the sun rose. Then he sat down until forenoon. Then he slept and woke before the sun came near to descending (from its midday zenith). Then he performed the ritual ablution and performed prayers until he performed the afternoon prayer; then he praised Allah until he performed the sunset prayer; then he performed a prayer between the sunset and the evening prayers; so this was his behavior till he died."These examples indicate that the Imam was very fond of worship and that he devoted himself to Allah. He was most times busy performing prayers; he prostrated himself in prayer so much that he had calluses like those of a camel; he had a boy and he ordered the boy to cut off those calluses from his forehead and nose; a poet has hinted at that, saying:
A callus became long out of his long sujud, so his forehead and his nose ulcerated.
He thought that his free time in prison was as his wish and as a favor for which he hanked Allah for a (long) time ( Bihar al-Anwar, vol. 11, p. 298. / Al-Anwar al-Bahiyya, p. 93. )