Musnad Abu Ya'la al-Mawsili 52:2

Zuhayr ibn Harb narrated to us, Abdullah ibn Yazid narrated to us, Haywah narrated to us, he said: Abu Hani’ Humayd ibn Hani’ al-Khawlani informed me that he heard Abu Abd al-Rahman al-Hubuli—who is Abdullah ibn Yazid—and Amr ibn Hurayth and others saying that the Messenger of Allah (may Allah bless him and grant him peace) said: "You will arrive at a people whose hair is curly, so treat them well, for they are a strength for you and a means of reaching your enemy, by the permission of Allah." He means the Copts of Egypt.