They i.e 'Ali and al-'Abbas (Allah be pleased with them), were quarrelling about what Allah bestowed on His Messenger of Allah ﷺ, that is, the property of Banu al-Nadir. Abu Dawud said: He ('Umar) intended that the name of division should not apply to it.
وَهُمَا يَعْنِي عَلِيًّا وَالْعَبَّاسَ ؓ يَخْتَصِمَانِ فِيمَا أَفَاءَ اللَّهُ عَلَى رَسُولِهِ مِنْ أَمْوَالِ بَنِي النَّضِيرِ