Musnad Ahmad 1060:59

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, he said: I heard Warqa' narrating from Sa'd ibn Sa'id, from 'Umar ibn Thabit, from Abu Ayyub, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever fasts Ramadan and six [days] of Shawwal, then he has fasted the [entire] time."