Musnad Ahmad 33:356

Muhammad ibn Bakr narrated to us, Ibn Jurayj informed us, Abu al-Zubayr informed me that he heard Jabir ibn Abdullah say: "The Messenger of Allah (peace and blessings of Allah be upon him) led us in prayer on the Day of Sacrifice (Yawm al-Nahr) in Medina. Two men stepped forward and slaughtered [their sacrifices], thinking that the Prophet (peace and blessings of Allah be upon him) had already slaughtered. The Prophet (peace and blessings of Allah be upon him) commanded whoever had slaughtered before him to repeat it with another sacrifice, and [commanded] that they should not slaughter until the Prophet (peace and blessings of Allah be upon him) had slaughtered."