Musnad Ahmad 1153:1431

I read to Abd al-Rahman: Malik, from Abdullah ibn Yazid and Abu al-Nadr, the freed slave of Umar ibn Ubayd Allah, from Abu Salamah ibn Abd al-Rahman, from Aisha, 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 while sitting. He would recite while he was sitting, and when there remained of his recitation the equivalent of thirty or forty verses, he would stand and recite while standing. Then he would bow, then he would prostrate, and then he would do the same in the second rak'ah."