Musnad Ahmad 30:2362

Isma'il ibn Ibrahim—who is Ibn 'Ulayyah—narrated to us, from Hisham ibn Hassan, and Yazid ibn Harun said: Hisham informed us, from Ibn Sirin, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever forgets while he is fasting, and so eats or drinks, let him complete his fast, for it is only Allah who has fed him and given him to drink."