Muwatta Malik Book 8, Hadith 24
Yahya related to me from Malik from Abdullah ibn Yazid al-Madani
and from Abu'n Nadr from Abu Salama ibn Abd ar-Rahman from A'isha, the
wife of the Prophet, may Allah bless him and grant him peace, that the
Messenger of Allah, may Allah bless him and grant him peace, used to
pray sitting. He would recite sitting, and then, when about thirty or
forty ayats of what he was reciting remained, he would stand up and
recite standing and then go into ruku and sajda. He would do the same
in the second raka.
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.
Yahya related to me from Malik that he had heard that Urwa ibn az-Zubayr and Said ibn al-Musayyab used to pray voluntary prayers sitting.