Sahih Muslim Book 4, Hadith Number 985.

Chapter : Following the Imam and acting after him.

Ibn Juraij reported: I asked ‘Ata’: What do you recite when you are in a state of bowing (in prayer)? He said: “Hallowed be Thou, and with Thy praise, there is no god but Thou.” Son of Abd Mulaika narrated to me on the authority of ‘Aisha (who reported): I missed one night the Apostle of Allah (may peace be upon him) (from his bed). I thought that he might have gone to one of his other wives. I searched for him and then came back and (found him) in a state of bowing, or prostration, saying: Hallowed be Thou and with Thy praise; there is no god but Thou. I said: With my father mayest thou be ransomed and with my mother. I was thinking of (another) affair, whereas you are (occupied) in another one.