Sahih al-Bukhari, Book 59, Hadith 110 (In-Book) ⓘ

Narrated `Uqba bin `Umar and Abu Mas`ud: Allah's Messenger 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."

حَدَّثَنَا مُسَدَّدٌ، حَدَّثَنَا يَحْيَى، عَنْ إِسْمَاعِيلَ، قَالَ حَدَّثَنِي قَيْسٌ، عَنْ عُقْبَةَ بْنِ عَمْرٍو أَبِي مَسْعُودٍ، قَالَ أَشَارَ رَسُولُ اللَّهِ صلى الله عليه وسلم بِيَدِهِ نَحْوَ الْيَمَنِ فَقَالَ ‏ "‏ الإِيمَانُ يَمَانٍ هَا هُنَا، أَلاَ إِنَّ الْقَسْوَةَ وَغِلَظَ الْقُلُوبِ فِي الْفَدَّادِينَ عِنْدَ أُصُولِ أَذْنَابِ الإِبِلِ، حَيْثُ يَطْلُعُ قَرْنَا الشَّيْطَانِ فِي رَبِيعَةَ وَمُضَرَ ‏" ‏‏.‏