Sahih Muslim 1704
صحيح مسلم#1704SahihA'isha reported:I did not see the Messenger of Allah (ﷺ) reciting (the Qur'an) in the night prayer in a sitting position, till he grew old and then he recited (it) in a sitting position, but when thirty or forty verses were left out of the Surah, he would then stand up, recite them and then bowed
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: 6:134