Musnad Ahmad 33:190

Abdullah ibn Yazid narrated to us, Sa’id—meaning Ibn Abi Ayyub—narrated to us, ‘Amr ibn Jabir al-Hadrami narrated to me, he said: I heard Jabir ibn Abdullah al-Ansari saying: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "Whoever fasts Ramadan and six days of Shawwal, it is as if he fasted the entire year."