Musnad Ahmad 32:1025

Isma'il narrated to us, Sa'id 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 horned rams. He would place his foot on their sides, slaughter them with his own hand, mention the name of Allah, and say the Takbir."