Musannaf Ibn Abi Shayba 27:795

Abu Usama narrated to us, from Isma'il, from Qays, from Abu Mas'ud, who said: The Messenger of Allah (may Allah bless him and grant him peace) gestured with his hand toward Yemen and said: "Indeed, Yemen is here. Verily, harshness and hardness of hearts are in the loud-voiced ones (al-faddadin) at the roots of the tails of camels, among Rabi'ah and Mudar."