Nothing heavier than good character
مَا مِنْ شَيْءٍ أَثْقَلُ فِي الْمِيزَانِ مِنْ حُسْنِ الْخُلُقِ
Ma min shay'in athqalu fi-l-mizani min husni-l-khuluq.
"Nothing is heavier in the scales [on the Day of Judgement] than good character."
Narrated by Abu Darda'. Sunan Abi Dawud 4799, Jami' at-Tirmidhi 2002 (sahih)
A short reflection
Heavier than prayer, heavier than charity, the way you treat people is weighed on a different scale.