Sahih Muslim Book 4, Hadith Number 1592.

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

Alqama b. Waqqas reported: I asked ‘Aisha how the Messenger of Allah (may peace be upon him) did in the two rak’ahs as he (observed them) sitting. She said: He would recite (the Qur’an) in them, and when he intended to bow, he would stand up and then bowed.