Sahih Bukhari: Hadith 1918

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?"