Sahih Bukhari Volume 003, Book 031, Hadith Number 204.
Narated By Mutarrif from 'Imran Ibn Husain : That the Prophet asked him (Imran) or asked a man and Imran was listening, "O Abu so-and-so! Have you fasted the last days of this month?" (The narrator thought that he said, "the month of Ramadan"). The man replied, "No, O Allah's Apostle!" The Prophet said to him, "When you finish your fasting (of Ramadan) fast two days (in Shawwal)." Through another series of narrators 'Imran said, "The Prophet said, '(Have you fasted) the last days of Sha'ban?"