Musnad Ahmad 33:269

Abu Mu'awiyah and Waki' narrated to us, they said: Al-A'mash narrated to us, from Abu Sufyan, from Jabir, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When one of you prostrates, let him be upright, and do not let him spread out his forearms like the spreading out of a dog."