Musnad Ahmad 32:869

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, and Hajjaj said: Shu'bah narrated to me, he said: I heard Qatadah narrating from Anas ibn Malik, from the Prophet, may Allah bless him and grant him peace, that he said: "Maintain moderation in prostration, and let not one of you spread out his forearms like the spreading out of a dog."