Hadith 1240
Lady Aisha (may Allah be pleased with her) narrates that the Noble Prophet (peace and blessings be upon him), after becoming old, used to pray while sitting. Then, when about thirty or forty verses of the surah remained, he (peace be upon him) would stand up and recite those verses, then go into bowing (ruku). Whereas in the narration of Ali bin Hujr (may Allah have mercy on him), the words are: "The Messenger of Allah (peace and blessings be upon him) did not recite while sitting in the Tahajjud prayer until his age increased (then he would recite while sitting)."
Hadith 1241
Hazrat Abdullah bin Shaqiq says that I asked Sayyida Aisha (may Allah be pleased with her), "Did the Messenger of Allah (peace and blessings be upon him) used to pray sitting?" She replied, "When the concerns of the people made the Messenger of Allah (peace and blessings be upon him) old, then after that he would pray (sitting)." The words of Janab Al-Dawraqi are that she said, "Yes, when the worries and concerns of the people made the Messenger of Allah (peace and blessings be upon him) old and weak, then after that (he would pray sitting)."