Hadith 517750
Chapter 54: General Subjects - كتاب الاستئذان
وَحَدَّثَنِي مَالِكٌ، أَنَّهُ بَلَغَهُ أَنَّ عُمَرَ بْنَ الْخَطَّابِ، أَرَادَ الْخُرُوجَ إِلَى الْعِرَاقِ فَقَالَ لَهُ كَعْبُ الأَحْبَارِ لاَ تَخْرُجْ إِلَيْهَا يَا أَمِيرَ الْمُؤْمِنِينَ فَإِنَّ بِهَا تِسْعَةَ أَعْشَارِ السِّحْرِ وَبِهَا فَسَقَةُ الْجِنِّ وَبِهَا الدَّاءُ الْعُضَالُ .
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."
Reference Muwatta Malik 517750
USC-MSA web (English) reference : Book 54, Hadith 30