Musnad Ahmad 32:20

Hushaym narrated to us, Shu'bah informed us, from Qatadah, Anas ibn Malik narrated to us, he said: "The Messenger of Allah (peace and blessings of Allah be upon him) used to sacrifice two horned, white-and-black rams. He would mention the name of Allah and say the takbir. I saw him slaughtering them with his own hand, placing his foot upon their sides."