Musnad Ahmad 30:3239

Muhammad ibn Ja'far narrated to us, Hisham narrated to us, from Muhammad, from Abu Hurairah, who said: The Messenger of Allah (ﷺ) said—or Abu al-Qasim said: "If one of you eats or drinks while forgetting, while he is fasting, then let him complete his fast, for it is only Allah who has fed him and given him drink."