Sahih Muslim 1706
صحيح مسلم#1706SahihA'isha reported:The Messenger of Allah (ﷺ) used to recite in sitting position (while observing the Tahajjud prayer) and when he intended to bow, he would stand up and recite (for the duration in which) a man (ordinarily) recites forty verses
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: 6:136