Musnad Ahmad 27:1316

'Affan narrated to us, Wuhayb narrated to us, Ayyub narrated to us, from Nafi', from Ibn 'Umar, from the Prophet (peace and blessings of Allah be upon him), who said: "If one of you is invited to a feast, let him answer," or he said: "Let him come to it." He said: And Ibn 'Umar used to answer while fasting and while not fasting.