Maliks Muwatta Book 8, Hadith Number 23.

Section : Praying Voluntary Prayers (Nawafil) Sitting.

Yahya related to me from Malik from Hisham ibn Urwa from his father that A’isha, the wife of the Prophet, may Allah bless him and grant him peace, told him that she had never seen the Messenger of Allah, may Allah bless him and grant him peace, doing night prayers sitting down until he was getting on in years. He would recite sitting down until when he wanted to go into ruku, he would stand up and recite about thirty or forty ayats and then go into ruku.