Musnad Ahmad 32:1259

'Abd al-Samad narrated to us, Hisham narrated to us, from Qatadah, from Anas, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) used to sacrifice two white-and-black rams with horns. He would slaughter them with his own hand, place his foot on their sides, and mention the name of Allah and say the Takbir."