Sahih Muslim Book 4, Hadith Number 1591.

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

‘Aisha reported: The Messenger of Allah (may peace be upon him) used to recite in sitting position (while observing the Tahajjud prayer) and when he intended to bow, he would stand up and recite (for the duration in which) a man (ordinarily) recites forty verses.