Sahih Muslim 1711
صحيح مسلم#1711SahihA'isha reported:When the Messenger of Allah (ﷺ) grew bulky and heavy he would observe (most of his Nafl) prayers sitting
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih Muslim is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 6:141