Musnad Ahmad 28:120

Hassan narrated to us, Ibn Lahi'ah narrated to us, Huyayy ibn Abdullah al-Ma'afiri narrated to me, that Abu Abd al-Rahman al-Hubuli narrated to him, from Abdullah ibn Amr: That a man came to the Prophet, peace and blessings of Allah be upon him, and said: "My father slaughtered his sacrifice before he prayed." The Messenger of Allah, peace and blessings of Allah be upon him, said: "Tell your father to pray, then slaughter."