Musnad Ahmad 32:793

Muhammad ibn Ja'far narrated to us, Sa'id narrated to us, from Qatadah, from Anas ibn Malik: "The Messenger of Allah (peace and blessings of Allah be upon him) used to sacrifice two white-and-black-colored, horned rams. He would slaughter them with his own hand, place his foot upon their sides, and mention the name of Allah."