Sahih Muslim Book 6, Hadith Number 2608.

Chapter : Fasting in the month of Sha’ban.

Imran b. Husain (Allah be pleased with them) reported that Allah’s Apostle (peace be upon him) 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 (may peace be upon him) said: Fast for two days instead of (one fast) when you have completed (fasts of) Ramadan.