Musannaf Ibn Abi Shayba 16:316

Abu Bakr narrated to us, he said: Abu Usama narrated to us, he said: Ubayd Allah ibn Umar narrated to us, from Nafi’, from Ibn Umar, he 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."