Sahih Muslim Book 4, Hadith Number 1585.

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

‘Aisha reported that the Messenger of Allah (may peace be upon him) would pray in the night for a long time, and when he prayed standing be bowed in a standing posture, and when he prayed sitting, he bowed in a sitting posture.