Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1590
'A'isha reported: The Messenger of Allah (may peace be upon him) used to pray while sitting (when he grew old) and he recited in this position and when the recitation equal to thirty or forty verses was left, he would then stand up and recite (for this duration) in a standing position and then bowed himself and then prostrated himself and did the same in the second rak'ah.