Imam Musa Kazim (a.s.) that people asked him: “Why seven Takbir were appointed before prayers and why in ruku are we supposed to recite: Glory be to my Lord the most great and praise be to Him and why we say Glory be to my Lord the most High and praise be to Him in Sajdah? Imam (a.s.) said: The Almighty Allah has created seven heavens seven levels of earth and seven curtains.
When the Messenger of Allah (S) went up to Meraj, he reached Qaba Qausain three times and from the seven curtains of Paradise one curtain was removed for the Prophet so he recited ‘Allahu Akbar’ (God is the Greatest), on removal of each curtain, he said: ‘Allahu Akbar’ (God is the Greatest); since prayer is the Meraj of believers, that is why it became recommended to recite seven Takbirs in the beginning.
So that the curtains that lie because of distance may be lifted from him. When after the removal of curtains from the heart of the Messenger of Allah (S) lights of greatness and majesty of Allah became illuminated for him, the Holy Prophet (S) began to tremble due to awe and he bowed down in Ruku and said: Glory be to my Lord the most great and praise be to Him, three times.
When he straightened, a Noor of that greatness shone on him and he fell down in prostration and recited seven times: Glory be to my Lord the most High and praise be to Him, and the awe that had come upon him was dispelled. That is why these recitations were made a part of Ruku and Sajdahs.