Musnad Ahmad 30:3534

Rawh narrated to us, Awf and Hisham narrated to us, from Muhammad, from Abu Hurairah, that the Messenger of Allah (peace and blessings of Allah be upon him) 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."