Sahih Muslim Book 4, Hadith Number 1588.

Chapter : The virtue of Sunan prayers before and after the Fard Rak’ahs and their number.

‘Abdullah b. Shaqiq al-‘Uqaili reported: I asked ‘Aisha 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.