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 him, or to another man: "Did you fast any part of the month of Sha'ban?" He said: "No." He (peace and blessings be upon him) said: "Then when you stop fasting (after Ramadan), fast two days."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 15310