Musannaf Ibn Abi Shayba 3:279

Mu'awiyah narrated to us, from 'Amr, he said: Za'idah narrated to us, from al-A'mash, and from Salih ibn Hubab, from Husayn ibn 'Uqbah, from 'Amr, from al-A'mash, from Abu Sufyan, from Jabir, from the Prophet (peace be upon him), he said: "When one of you prostrates, let him be upright, and let him not spread his forearms like the spreading of a dog."