It is narrated from Sayyiduna Imran bin Husayn (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said to a man: "Did you fast in the last days of this month, meaning Sha'ban?" He said: "No." He (peace and blessings be upon him) said: "Then when you have completed (the fasts of) Ramadan and broken your fast, fast one or two days."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 15332