Muwatta Malik Book 8, Hadith 23
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 from Ibn Shihab from as-Sa'ib ibn Yazid from al Muttalib ibn Abi Wadaa as-Sahmi that Hafsa, the wife of the Prophet, may Allah bless him and grant him peace, said, "I never saw the Messenger of Allah, may Allah bless him and grant him peace, praying nawafil sitting, until a year before his death, when he began to pray them sitting. He would recite the sura with a measured slowness so that it would seem to be longer than other suras which were actually longer than it."
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.