Musnad Abu Ya'la al-Mawsili 155:196

Abu Bakr narrated to us, Hafs narrated to us, from Hisham, from Ibn Sirin, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "If one of you is invited, let him answer. If he is fasting, let him pray (for the host), and if he is not fasting, let him eat."