Musnad Ahmad 790:132

Hashim and ‘Affan narrated to us, they said: Mahdi narrated to us, he said: ‘Affan said: Ghaylan narrated to us, from Mutarrif, from ‘Imran ibn Husayn, from the Prophet (may Allah bless him and grant him peace)—either he said it to ‘Imran or to a man while he was listening—: "Did you fast the *surar* (the end days) of this month?" He said: "No." He said: "Then when you break your fast, fast two days."