Mishkat al-Masabih 3915
‘Abdallah b. Mas’ud said:
At the battle of Badr there was one camel to every three men of us. Abu Lubaba and ‘Ali b. Abu Talib were the travelling companions of God’s Messenger, and when his turn to dismount came they would offer to walk instead of him, but he would reply, “You are not stronger than I am, and I am not more able to dispense with the reward* than you are." 

* i.e. the reward in the next world for walking part of the way.

It is transmitted in Sharh as-sunna .
وَعَنْ عَبْدِ اللَّهِ بْنِ مَسْعُودٍ رَضِيَ اللَّهُ عَنْهُ قَالَ: كُنَّا يَوْمَ بَدْرٍ كُلَّ ثَلَاثَةٍ عَلَى بَعِيرٍ فَكَانَ أَبُو لُبَابَةَ وَعَلِيُّ بْنُ أَبِي طَالِبٍ زَمِيلَيْ رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ: فَكَانَتْ إِذَا جَاءَتْ عُقْبَةُ رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَا: نَحْنُ نَمْشِي عَنْكَ قَالَ: «مَا أَنْتُمَا بِأَقْوَى مِنِّي وَمَا أَنَا بِأَغْنَى عَنِ الْأَجْرِ مِنْكُمَا» . رَوَاهُ فِي شرح السّنة