Musnad Ahmad 30:3218

Muhammad ibn Ja‘far narrated to us, he said: Sa‘id narrated to us, from Qatadah, that Abu Rafi‘ narrated from Abu Hurayrah, that the Messenger of Allah (may Allah bless him and grant him peace) said: "Whoever eats or drinks while fasting out of forgetfulness, let him complete his fast, for Allah, the Almighty and Majestic, has fed him and given him drink."