The Book of Prayers (Kitab Al-Salat)
Muslim :: Book 4 : Hadith 1588
'Abdullah b. Shaqiq al-'Uqaili reported: I asked 'A'isha about the prayer of the Messenger of Allah (may peace be upon him). She said: The Messenger of Allah (may peace be upon him) would observe prayer (Nafl) in a standing position as well as in a sitting position, and when he commenced the prayer in a standing position, he bowed in this very position, and when he commenced the prayer in a sitting position, he bowed in this very position.