Sahih Muslim 2751
صحيح مسلم#2751SahihImran b. Husain (Allah be pleased with them) reported Allah's Messenger (may peace he upon him) having said to him or to someone else:Did you fast in the middle of Sha'ban? He said: No. Thereupon he (the Holy Prophet) said: If you did not observe fast, then you should observe fast for two days
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: 13:257