Sahih Bukhari Volume 7, Book 63, Hadith Number 223.

Narrated By Abu Masud : The Prophet pointed with his hand towards Yemen and said twice, “Faith is there,” and then pointed towards the East, and said, “Verily, sternness and mercilessness are the qualities of those who are busy with their camels and pay no attention to their religion, where the two sides of the head of Satan will appear,” namely, the tribes of Rabl’a and Muqar.