Musnad Ahmad 32:1737

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, he said: I heard Qatadah narrating from Anas ibn Malik: "The Messenger of Allah, peace and blessings of Allah be upon him, used to sacrifice two white-and-black rams with horns, and he would say the takbir. And I indeed saw him slaughtering them with his own hand, placing his foot upon their sides."