Musnad Ahmad 1060:36

Abu Mu'awiyah narrated to us, Sa'd bin Sa'id narrated to us, from 'Umar bin Thabit, from Abu Ayyub al-Ansari, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever fasts Ramadan, then follows it with six days of Shawwal, that is the fasting of a lifetime."