Musnad Ahmad 32:2029

Abdullah narrated to us, Abdullah ibn Sa’d narrated to me, he said: My uncle Ya’qub narrated to me, from Sharik, from Shu’bah, from Qatadah, from Anas, from the Prophet, peace and blessings of Allah be upon him, who said: "Maintain balance in your prostration, and let none of you spread out his forearms like the spreading out of a dog. Complete the bowing and the prostration. For by Allah, I surely see you from behind me—or from behind my back—when you bow and when you prostrate."