Imran b. Husain (Allah be pleased with them) reported Allah's Messenger (ﷺ) 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.
Reference: Sahih Muslim 1161 b In-book : Book 13, Hadith 257 USC-MSA web (English) : Book 6, Hadith 2607 (deprecated)
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