Musnad Ahmad 26:459

Ishaq ibn Isa and Hassan ibn Musa narrated to us, they said: Hammad ibn Salamah narrated to us, from Asim ibn Bahdalah, from Zirr ibn Hubaysh, from Abdullah ibn Mas’ud, who said: We were at the Battle of Badr, and every three of us were upon one camel. Ali and Abu Lubabah were the companions of the Messenger of Allah (peace and blessings of Allah be upon him) on his camel. When it was the turn of the Prophet (peace and blessings of Allah be upon him) 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 at walking than I am, and I am not in less need of the reward than you two."