Musannaf Ibn Abi Shayba 3:274

Hafs ibn Ghiyath, Abu Mu'awiyah, and Abu Khalid al-Ahmar narrated to us, from al-A'mash, from Abu Sufyan, from Jabir, who said: The Prophet (peace and blessings of Allah be upon him) said: "When one of you prostrates, let him be balanced (straighten himself), and let him not spread out his forearms like the spreading out of a dog."