Musnad Ahmad 790:156

Yazid narrated to us, Sulayman al-Taymi informed us, from Abu al-Ala’ ibn al-Shikhkhir, from Imran ibn Husayn—and Sulayman said: "I am in doubt regarding Imran"—that the Prophet, peace and blessings of Allah be upon him, said to him: "O Imran, have you fasted anything from the *surar* (end/secret days) of this month?" He said: "No." He said: "Then when you break your fast, fast two days in its place." And Ibn Abi Adi said: *Sarar*.