Musnad Ahmad 30:3262

Muhammad ibn Ja'far narrated to us, he said: 'Awf narrated to us, from al-Hasan, he said: It reached me that the Messenger of Allah, may Allah bless him and grant him peace, said: "If one of you is fasting and he forgets, then eats and drinks, let him complete his fast, for Allah, the Mighty and Majestic, has fed him and given him drink."