Musnad Ahmad 790:191

'Abd al-Samad narrated to us, Mahdi narrated to us, Ghaylan narrated to us, from Mutarrif, from 'Imran ibn Husayn, that the Prophet (peace and blessings of Allah be upon him) asked him, or asked a man while he was present: "Have you fasted anything from the *surar* (the end days) of this month?" He said: "No." He said: "Then when you break your fast, fast two days."