Musnad Ahmad 26:479

Abu Kamil narrated to us, Hammad narrated to us, from Asim ibn Bahdalah, from Zirr ibn Hubaysh, from Ibn Mas'ud, that the Messenger of Allah (peace and blessings of Allah be upon him) had as his riding companions on the day of Badr, Ali and Abu Lubabah. When the turn of the Messenger of Allah (peace and blessings of Allah be upon him) came to walk, they both said: "Ride, O Messenger of Allah, so that we may walk for you." He would say: "You two are not stronger than me, and I am not more in need of the reward than you two."