Jundub bin Sufyan al-Bajali (may Allah be pleased with him) said: I was with the Prophet (peace and blessings be upon him) on the day of sacrifice. He (peace and blessings be upon him) said: "Whoever slaughters before offering the prayer, let him sacrifice another animal in its place after praying; and whoever has not yet slaughtered, let him slaughter."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19022