Sahih Muslim
The Book of Fasting (Kitab Al-Sawm)
Book Number: 006 / Hadith Number: 2608
Imran b. Husain (Allah be pleased with them) reported that Allah's Apostle (way peace heupon 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.