Sunan an-Nasa'i 1649
سنن النسائي#1649SahihIt was narrated that Aishah said:"I never saw the Messenger of Allah (ﷺ) pray sitting down until he grew old. Then he would pray sitting down and when there were thirty or forty verses left, he would stand up and recite them, then bow
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Abu GhuddahSahih
Al-AlbaniSahih
Zubair Ali ZaiSahih - Agreed Upon
In-book reference: 20:52