Musnad Abu Ya'la al-Mawsili 152:291

Abu Khaythama narrated to us, Affan narrated to us, Hammad bin Salamah narrated to us, Asim bin Bahdalah informed us, from Zirr, from Abdullah bin Mas'ud, who said: We were on the day of Badr, every three [men] upon one camel. Abu Lubabah and Ali bin Abi Talib were the companions of the Messenger of Allah, may Allah bless him and grant him peace. He said: When the turn of the Messenger of Allah, may Allah bless him and grant him peace, came [to walk], 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."