Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1587
'Abdullah b. Shaqiq al-'Uqaili reported: I asked 'A'isha about the prayer of the Messenger of Allah (may peace be upon him) during the night (i. e. Tahajjud prayer) She replied: He used to pray for a long time standing and for a long time sitting in the night, and when he recited the Qur'an while standing, he would bow himself from the standing position, and when he recited while sitting, he would bow from the sitting position.