Sunan an-Nasa'i 1658
سنن النسائي#1658SahihIt was narrated that Hafsah said:"I never saw the Messenger of Allah (ﷺ) offer his voluntary prayers sitting down until one year before his death. Then he used to pray sitting down, reciting the surah so slowly that it seemed to be longer than a surah that is longer
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 Muslim
In-book reference: 20:61