Al-Aswad relates: I heard Sayyiduna Jundub (may Allah be pleased with him) say: I was present with the Prophet (peace and blessings be upon him); then he (peace and blessings be upon him) delivered a sermon and said: "Whoever slaughtered his sacrifice before the prayer, let him slaughter another animal in its place, and whoever has not yet slaughtered, let him slaughter in the name of Allah."
Hadith Referenceالمعجم الكبير للطبراني / باب الجيم / 1700