Musnad Ahmad 32:1291

'Abd al-Malik narrated to us, Hisham narrated to us, from Qatadah, from Anas: "The Prophet, peace and blessings of Allah be upon him, sacrificed two horned, black-and-white rams. He slaughtered them with his own hand, mentioned the name of Allah (said Bismillah), said the Takbir (Allahu Akbar), and placed his foot on their sides."