Musnad Ahmad 30:3227

Muhammad ibn Ja'far narrated to us, he said: Mu'ammar narrated to us, he said: Yahya ibn Abi Kathir informed us, from Damdam, from Abu Hurairah, he said: "The Messenger of Allah, may Allah bless him and grant him peace, commanded the killing of the two black ones during prayer." I said to Yahya: "What does he mean by the two black ones?" He said: "The snake and the scorpion."