People asked Imam Musa Kazim (a.s.): “Why one Ruku and two sajdahs were made obligatory in each rakat of prayer?” Imam (a.s.) said: “The first prayer that the Holy Prophet (S) performed was in front of the Divine Throne. Because on the night of Meraj, the Messenger of Allah (S) was carried to the heavens and when he reached below the Arsh, the Almighty Allah said to him: “O Muhammad, wash your face and hands at the stream of Saad and pray for your Lord.
The Holy Prophet (S) went to that spring and performed the complete ablution, and stood up to pray. The Almighty Allah commanded him to initiate the prayer and the Holy Prophet (S) recited the Takbir. He was told to recite from In the name of Allah, the Beneficent, the Merciful to the end of Surah Hamd. Then recite Surah Tauheed.
The Messenger of Allah (S) after these recitations said: “As my Lord is”. He was ordered to perform ruku and he obliged and the Almighty Allah told him to recite: Glory be to my Lord who is most great and praise be to Him. Which the Prophet said thrice and then he was told to raise his head. The Holy Prophet (S) stood erect.
He was then told to prostrate to his Lord. When he prostrated he was ordered to recite: Glory be to my Lord who is most High and praise be to Him. The Messenger of Allah (S) recited it thrice and the Almighty Allah said: O Muhammad, now sit up straight. His Eminence, sat up and mentioned the greatness and majesty of his Lord and then by the order of the Lord of the Lords went into prostration again and recited the Tasbih thrice.
Then he was told to stand and recite the chapters of Qur’an. Again he was ordered to perform Ruku and Sajdah. When he performed the first Sajdah, he mentioned about the majesty of his Lord and then performed the second Sajdah. The Almighty Allah said: Raise your head from the Sajdah, the Almighty Allah will exalt you. Now recite the Tashahud. When the Prophet concluded the Tashahud he was told to recite salutation to his Lord.
The Almighty Allah said in reply: And peace be upon you. O Muhammad, you have received the bounty of worshipping Me along with the other bounties. I have bestowed you with prophethood with My infallibility and made you My beloved. Imam Musa Kazim (a.s.) says: In each rakat, the Almighty Allah commanded one Ruku and one Sajdah, the Holy Prophet (S) because of the imagination of the greatness of Allah, added another Sajdah and the Almighty Allah made that also obligatory.