Musnad Ahmad 30:2493

Yahya narrated to us, from Muhammad ibn ‘Amr, and Yazid said: Muhammad ibn ‘Amr informed us, saying: Abu Salamah narrated to us, from Abu Hurairah, from the Prophet, peace and blessings of Allah be upon him, who said: "The believer eats in one intestine, and the disbeliever eats in seven intestines."