Al-Bara bin Azib (may Allah be pleased with him) says that the Prophet (peace and blessings be upon him) said: "Whoever sacrifices before the prayer has slaughtered only for himself, and whoever slaughters the animal after the prayer, his sacrifice is complete and he has attained the way of the Muslims."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19114