Sahih Muslim 1681
صحيح مسلم#1681SahihA'isha reported: The Messenger of Allah (ﷺ) used to observe two rak'ahs of Sunnah (prayer) when he heard the Adhin and shortened them
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: 6:111