Maliks Muwatta Book 8, Hadith Number 24.
Section : Praying Voluntary Prayers (Nawafil) 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.