Muwatta Malik Book 8, Hadith 25
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.
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 Zayd ibn Aslam from al-Qaqa ibn Hakim that Abu Yunus, the mawla of A'isha, umm al-muminin said, ''A'isha ordered me to write out a Qur'an for her. She said, 'When you reach this ayat, let me know, "Guard the prayers carefully and the middle prayer and stand obedient to Allah." ' When I reached it I told her, and she dictated to me, 'Guard the prayers carefully and the middle prayer and the asr prayer and stand obedient to Allah.' A'isha said, 'I heard it from the Messenger of Allah, may Allah bless him and grant him peace.' "