Musnad Ahmad 32:207

Yahya narrated to us, from Shu'bah, Qatadah narrated to us, from Anas: "That the Prophet, peace and blessings of Allah be upon him, sacrificed two horned, black-and-white rams. I indeed saw him slaughtering them with his hand, placing his foot upon their sides, and he would mention the name of Allah and say the takbir."