Sahih Bukhari Volume 4, Book 54, Hadith Number 521.

Narated By ‘Uqba bin ‘Umar and Abu Mas’ud : Allah’s Apostle pointed with his hand towards Yemen and said, “True Belief is Yemenite yonder (i.e. the Yemenite, had True Belief and embraced Islam readily), but sternness and mercilessness are the qualities of those who are busy with their camels and pay no attention to the Religion where the two sides of the head of Satan will appear. Such qualities belong to the tribe of Rabi’a and Mudar.”