Musnad Ahmad 32:1770

'Affan and Bahz narrated to us, they said: Hammam narrated to us, Qatadah narrated to us, from Anas: "The Prophet, may Allah bless him and grant him peace, used to sacrifice two white-and-black horned rams. He would place his foot on their sides, slaughter them with his own hand, and mention the name of Allah and say the takbir."