Musnad Ahmad 1153:1825

Ishaq ibn Yusuf narrated to us, he said: Sufyan informed us, from Abdullah ibn Muhammad ibn Aqil, from Abu Salamah, from Abu Hurairah, that Aisha said: When the Messenger of Allah (peace and blessings of Allah be upon him) would offer a sacrifice, he would buy two large, fat, horned, black-and-white, castrated rams. He said: He would slaughter one of them on behalf of his nation, for those who affirmed the Oneness of Allah and bore witness to his delivery of the message, and he would slaughter the other on behalf of Muhammad and the family of Muhammad.