Musnad Ahmed: Hadith 3940

On the day of Badr, they had one camel for three men, and the ones who rode with the Prophet (ﷺ) were ‘Ali and Abu Lubabah. When it was the Prophet’s turn to walk, they said to him: Ride, and we will walk instead of you. He said: "You are not stronger than me and I am not in less need of reward than you.”