Sahih Bukhari

Narrated Mutarrif from `Imran Ibn Husain:That the Prophet (ﷺ) 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 Messenger (ﷺ)!"" The Prophet (ﷺ) said to him, ""When you finish your fasting (of Ramadan) fast two days (in Shawwal)."" Through another series of narrators `Imran said, ""The Prophet (ﷺ) said, '(Have you fasted) the last days of Sha'ban?