Sayyiduna Anas bin Malik (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) delivered a sermon after the prayer and commanded: "Whoever slaughtered his animal before the prayer should slaughter another in its place." A man from the Ansar stood up and said: "My neighbours were suffering from hunger, so I slaughtered before the prayer. But I have a young goat that I like more than two goats of meat." So he (peace and blessings be upon him) granted him a concession. If it was a concession for him alone, then so it is; otherwise I have no knowledge. Then he (peace and blessings be upon him) slaughtered his two rams, and the people went and slaughtered their goats.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19116