Musnad Ahmad 1153:1868

'Abd al-Razzaq narrated to us, he said: al-Thawri informed us, from 'Abd Allah ibn Muhammad ibn 'Aqil, from Abu Salamah, from 'Aishah, or from Abu Hurayrah, that the Prophet, peace and blessings of Allah be upon him, when he intended to offer a sacrifice, would buy two large, fat, white-and-black, horned, castrated rams. He would slaughter one of them on behalf of his nation, for whoever testified to the Oneness (of Allah) and testified to his delivery (of the message), and he would slaughter the other on behalf of Muhammad, peace and blessings of Allah be upon him, and the family of Muhammad.