Musnad Abu Ya'la al-Mawsili 109:286

Ibn Numayr narrated to us, my father narrated to us, from al-A‘mash, from Abu Sufyan, from Jabir, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "When one of you prostrates, let him be balanced, and let him not spread out his forearms like the spreading out of a dog."