Al-Aswad bin Qays narrates that Sayyiduna Jundub bin Sufyan (may Allah be pleased with him) said: I was present with the Prophet (peace and blessings be upon him) on the day of Eid al-Adha, and he (peace and blessings be upon him) saw that some people had slaughtered before the prayer. So he said: "Whoever slaughtered before the prayer, let him slaughter again; and whoever has not slaughtered, let him slaughter with the name of Allah."
Hadith Referenceالمعجم الكبير للطبراني / باب الجيم / 1703