Musannaf Ibn Abi Shayba 28:1019

Ghundar narrated to us, from Shu'bah, from Simak, who said: I heard 'Iyad al-Ash'ari say: I witnessed Yarmouk. He said: Abu Ubaydah ibn al-Jarrah said, "Who will race me?" He said: A young man said, "I, if you do not get angry." He said: So he raced him. He said: And I saw the two braids of Abu Ubaydah bouncing while he was behind him on an Arabian horse.