Musannaf Ibn Abi Shayba 11:220

Ibn Abi Ghaniyah narrated to us, from his father, from Isma'il bin Abi Khalid, from Qays or someone else—the doubt is from him—who said: Abu Bakr sent an army to the Levant, and he went out walking on his feet to see them off. They said, "O Caliph of the Messenger of Allah, if only you would ride." He said, "I seek the reward for my footsteps in the path of Allah."