Musnad Abu Ya'la al-Mawsili 109:9

Narrated to us Ishaq, narrated to us Jarir, from al-A'mash, from Abu Sufyan, from Jabir, who said: I heard the Prophet, peace and blessings of Allah be upon him, saying: "When one of you prostrates, let him be balanced (straighten his posture), and let him not spread out his forearms like the spreading out of a dog."