Musnad Ahmad 30:631

'Abd al-Razzaq narrated to us, Hisham narrated to us, from Muhammad, from Abu Hurayrah, who said: I heard the Prophet, peace and blessings of Allah be upon him, saying: "Whoever is invited, let him answer. If he is not fasting, let him eat, and if he is fasting, let him pray (supplicate) and invoke blessings for them."