Musnad Ahmad 33:1061

'Abd al-Razzaq narrated to us, Sufyan informed 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 upright, and let him not spread out his forearms like the spreading out of a dog."