Abu ad-Darda' (RAA) narrated that the Messenger of Allah (ﷺ) said:
"The heaviest thing which will be put on the believer's scale (on the Day of Resurrection) will be good morals." Related by Abu Dawud and At-Tirmidhi who graded it as Sahih.
English reference: Book 16, Hadith 1566
Arabic reference: Book 16, Hadith 1523