Sahih Al Bukhari Hadith Number 1119

Narrated `Aisha: (the mother of the faithful believers) Allah’s Messenger (peace be upon him) (in his last days) used to pray sitting. He would recite while sitting, and when thirty or forty verses remained from the recitation he would get up and recite them while standing and then he would bow and prostrate. He used to do the same in the second rak`a. After finishing the Prayer he used to look at me and if I was awake he would talk to me and if I was asleep, he would lie down.

18  Shortening the Prayers (At-Taqseer)
Hadith number 1119
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)