Al-Sha'bi narrates from Hudhayfah (may Allah be pleased with him): The Prophet (peace and blessings be upon him) would have two piebald rams brought near. He would slaughter one of them and say: "O Allah, this is on behalf of Muhammad and the family of Muhammad"; and he would have the other brought near and say: "O Allah, this is on behalf of everyone of my Ummah who bears witness to Your Oneness and bears witness that I have conveyed the message."
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 3046