Musnad Ahmad 790:163

Rawh narrated to us, Hammad narrated to us, from Thabit, from Mutarrif, from Imran ibn Husayn, that the Prophet (peace and blessings of Allah be upon him) said to him or to someone else: "Have you fasted anything from the *surar* (end/middle) of Sha'ban?" He said: "No." He said: "Then when you break your fast, fast two days."