Sayyidah Aishah or Sayyiduna Abu Hurayrah (may Allah be pleased with them) said: The Messenger of Allah (peace and blessings be upon him) used to sacrifice two horned, castrated rams. He would begin with one of them, saying: 'Bismillah wallahu akbar' (In the name of Allah, and Allah is the Greatest), "O Allah, it is from You and for You, on behalf of Muhammad and his Ummah, those who bore witness to You of Your Oneness and bore witness to me of conveying (the message)." And he would slaughter the other, saying: 'Bismillah wallahu akbar' (In the name of Allah, and Allah is the Greatest), "O Allah, it is from You and for You, on behalf of Muhammad and the family of Muhammad."
(b) In the narration of Ibn Abdan: Whenever the Prophet (peace and blessings be upon him) sacrificed, he would buy two fat, horned, black-and-white, castrated rams. He would slaughter one of them on behalf of his Ummah, those who bore witness to Allah's Oneness and bore witness to his conveying (the message), and the other on behalf of Muhammad and the family of Muhammad.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19185