Musnad Ahmad 790:155

Yazid narrated to us, [saying] Sa'id al-Jurayri informed us, from Abu al-'Ala', from Mutarrif, from 'Imran ibn Husayn, that the Prophet (peace and blessings of Allah be upon him) said to a man: "Have you fasted any of the *sarar* (the end days) of this month?" He said: "No." The Messenger of Allah (peace and blessings of Allah be upon him) said: "Then when you break your fast from Ramadan, fast two days in its place."