Sayyidah Aishah (may Allah be pleased with her) said that the Messenger of Allah (peace and blessings be upon him) ordered a horned ram with black legs, a black belly and black around its eyes, and it was brought to him to be sacrificed. He (peace and blessings be upon him) said: "O Aishah, bring the knife and sharpen it on a stone." I did so. Then he took it, took hold of the ram, laid it down and began to slaughter it, and said: 'Bismillah, Allahumma taqabbal min Muhammadin wa ali Muhammadin wa min ummati Muhammad' (In the name of Allah; O Allah, accept from Muhammad, the family of Muhammad and the Ummah of Muhammad). Then he sacrificed it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19182