Sayyiduna Abu Hurayrah (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "When one of you forgets and eats or drinks while he is fasting, let him complete his fast, for it is only Allah who has fed him and given him to drink."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8071