(37) باب صَوْمِ سَرَرِ شَعْبَانَ
'Imran 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.
Reference: Sahih Muslim 1161 b
In-book reference: Book 13, Hadith 257
USC-MSA web (English) reference(deprecated numbering scheme): Book 6, Hadith 2607