Maliks Muwatta Book 49. The Description Of Prophet (PBUH)
Section : The Intestines of the Kafir.
Yahya related to me from Malik from Abu'z-Zinad from al-Araj that Abu Hurayra said, "The Messenger of Allah, may Allah bless him and grant him peace, said, 'The muslim eats in one intestine, and the kafir eats in seven!'"
