Maliks Muwatta Book 54, Hadith Number 30.

Malik related to me that he heard that Umar ibn al-Khattab wanted to go to Iraq, and Kabal-Ahbar said to him, “Do not go there, amir al-muminin. There is nine-tenths of sorcery there and it is the place of the rebellious jinn and the disease which the doctors are unable to cure.”