Musnad Ahmad 26:351

'Affan narrated to us, Hammad ibn Salamah narrated to us, 'Asim ibn Bahdalah informed us, from Zirr ibn Hubaysh, from 'Abdullah ibn Mas'ud, who said: We were on the day of Badr, every three [men] upon one camel. Abu Lubabah and 'Ali ibn Abi Talib were the companions of the Messenger of Allah, peace and blessings of Allah be upon him, [on his camel]. He said: It was the turn of the Messenger of Allah, peace and blessings of Allah be upon him, [to walk]. He said: So they both said, "We will walk for you." He said: "You two are not stronger than me, and I am not more in need of the reward than you two."