Musnad Abu Ya'la al-Mawsili 155:231

Al-Mu'alla ibn Mahdi narrated to us, Imran ibn Khalid narrated to us, from Muhammad ibn Sirin, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever forgets and eats or drinks, let him complete his fast, for it is only Allah who has fed him and given him drink."