Musannaf Ibn Abi Shayba 16:318

Abu Bakr narrated to us, he said: Muhammad bin Kathir narrated to us, from Muhammad bin ‘Amr, from Abu Salamah, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The believer eats in one intestine, and the disbeliever eats in seven intestines."