(37) باب صَوْمِ سَرَرِ شَعْبَانَ
Imran 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.
Reference: Sahih Muslim 1161 c
In-book reference: Book 13, Hadith 258
USC-MSA web (English) reference(deprecated numbering scheme): Book 6, Hadith 2608