Musnad Ahmad 2:262

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Simak, who said: I heard 'Iyad al-Ash'ari say: I witnessed the Battle of Yarmouk, and we had five commanders over us: Abu Ubaydah ibn al-Jarrah, Yazid ibn Abi Sufyan, Ibn Hasanah, Khalid ibn al-Walid, and 'Iyad—and this 'Iyad is not the one who narrated to Simak. He said: Umar had said, "If there is fighting, then Abu Ubaydah is in command over you." He said: So we wrote to him that death had surged toward us, and we sought reinforcements from him. He wrote back to us: "Your letter has reached me, in which you seek my help, and I guide you to One who is mightier in victory and more present in troops: Allah, the Mighty and Majestic. So seek victory from Him, for Muhammad, peace and blessings of Allah be upon him, was granted victory on the day of Badr with fewer than your number. So when this letter of mine reaches you, fight them, and do not refer back to me." He said: So we fought them and defeated them, and we pursued them for four leagues. He said: We obtained wealth, and they consulted one another. 'Iyad suggested to us that we give ten [dirhams] for every head. He said: Abu Ubaydah said, "Who will race me?" A young man said, "I will, if you do not get angry." He said: So he outpaced him, and I saw the two braids of Abu Ubaydah bouncing while he was behind him on an Arabian horse.