Musnad Ahmad 26:415

'Abd al-Samad narrated to us, Hammad narrated to us, from 'Asim, from Zirr, from Ibn Mas'ud, who said: On the day of Badr, there was one camel for every three men. The companions of the Prophet, peace and blessings of Allah be upon him, were 'Ali and Abu Lubabah. He said: When it was the turn of the Prophet, peace and blessings of Allah be upon him, to walk, they both said to him: "Ride, so that we may walk for you." He would say: "You two are not stronger than me, and I am not in less need of the reward than you two."