Sayyidah Aishah or Sayyiduna Abu Hurayrah (may Allah be pleased with them both) said that two piebald, horned, castrated rams were brought to the Messenger of Allah (peace and blessings be upon him), and he (peace and blessings be upon him) slaughtered them: one on behalf of those who affirm Allah's Oneness and testify to his (peace and blessings be upon him) having conveyed (the message), and the other on behalf of Muhammad (peace and blessings be upon him) and the family of Muhammad (peace and blessings be upon him).
In the narration of al-Firyabi: When he (peace and blessings be upon him) intended to sacrifice, he would buy fat, horned, piebald, castrated rams.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19047