Musnad Ahmad 1060:64

Ibn Numayr narrated to us, Sa‘d ibn Sa‘id al-Ansari, the brother of Yahya ibn Sa‘id, narrated to us, ‘Umar ibn Thabit, a man from Banu al-Harith, informed me, Abu Ayyub al-Ansari informed me, he said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "Whoever fasts Ramadan, then follows it with six from Shawwal, then that is the fasting of the [entire] age."