Musnad Ahmad 32:1952

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