Sahih Muslim 2752
صحيح مسلم#2752SahihImran b. Husain (Allah be pleased with them) reported that Allah's Apostle (ﷺ) said to a person:Did you observe any fast in the middle of this month (Sha'ban)? He said: No. Thereupon the Messenger of Allah (ﷺ) said: Fast for two days instead of (one fast) when you have completed (fasts of) Ramadan
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: 13:258