Sahih Al Bukhari Hadith Number 1118

Narrated Aisha: (the mother of the faithful believers) I never saw Allah’s Messenger (peace be upon him) offering the night prayer while sitting except in his old age and then he used to recite while sitting and whenever he wanted to bow he would get up and recite thirty or forty verses (while standing) and then bow.

18  Shortening the Prayers (At-Taqseer)
Hadith number 1118
Volume no 2
Chapter: 20 (Whoever starts his Salat sitting (because of ailment) and then during the Salat (prayer) feels better, can finish the rest while standing)