Musnad Ahmad 30:3219

Muhammad ibn Ja'far narrated to us, he said: Sa'id narrated to us, from Ayyub, from Muhammad ibn Sirin, from Abu Hurairah, from the Prophet, peace and blessings of Allah be upon him, who said: "If one of you is invited, let him answer. If he is fasting, let him pray (make supplication)." Meaning: the supplication.