Sahih Muslim Book 6, Hadith Number 2609.

Chapter : Fasting in the month of Sha’ban.

‘Imran b. Husain (Allah be pleased with them) reported that the Apostle of Allah (may peace be upon him) said to a person: Did you observe fast in the middle of this month i.e. Sha’ban? He said: No. Thereupon he said to him: When it is the end of Ramadan, then observe fast for one day or two (Shu’ba had some doubt about it) but he said: I think that he has said: two days.