Musnad Abu Ya'la al-Mawsili 116:2

Ahmad narrated to us, Bahz and Wahb ibn Jarir narrated to us, they said: Shu'bah narrated to us, from Qatadah, from Anas: That the Messenger of Allah (ﷺ) said: "Maintain balance in prostration, and let not one of you spread out his forearms as the dog spreads them out."