Musnad Ahmad 980:5

Yahya narrated to us, from Isma'il, Qays narrated to us, from Abu Mas'ud, who said: The Messenger of Allah, peace and blessings of Allah be upon him, gestured with his hand toward Yemen and said: "Faith is here, faith is here. And indeed, harshness and hardness of hearts are in the loud-voiced ones, at the roots of the tails of camels, where the two horns of Satan emerge, in Rabi'ah and Mudar."