Sahih Muslim 1663
صحيح مسلم#1663SahihMu'adha asked 'A'isha (Allah be pleased with her) how many rak'ahs Allah's Messenger (ﷺ) prayed at the forenoon prayer. She replied:Four rak'ahs, but sometimes more as he pleased
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: 6:93