Sayyiduna Abu Rafi' (may Allah be pleased with him) said: When the Messenger of Allah (peace and blessings be upon him) intended to offer a sacrifice, he would buy two horned, black-and-white rams. When he had prayed and delivered the sermon, he would call for one of them at his place of prayer and slaughter it, then say: "O Allah, this is on behalf of my entire Ummah, whoever has testified to Your Oneness and testified that I conveyed the message." Then he would come to the other and slaughter it, then say: "O Allah, this is on behalf of Muhammad and his household."
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 918