Musnad Ahmad 33:593

Abdullah ibn Yazid narrated to us, Sa'id ibn Abi Ayyub narrated to us, from 'Amr ibn Jabir al-Hadrami, from Jabir ibn Abdullah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever fasts Ramadan and six days of Shawwal, it is as if he fasted the entire year."