Musnad Ahmad 1018:94

'Abd al-Rahman ibn Mahdi narrated to us, Zuhayr—meaning Ibn Muhammad—narrated to us, from Musa ibn Jubayr, from Abu Umamah ibn Sahl ibn Hunayf, who said: I heard a man from the companions of the Prophet, may Allah bless him and grant him peace, saying: I heard the Messenger of Allah, may Allah bless him and grant him peace, saying: "Leave the Abyssinians alone as long as they leave you alone, for none shall extract the treasure of the Ka'bah except the one with the two thin shins from the Abyssinians."