Musnad Ahmad 1302:43

Sufyan narrated to us, again from ‘Amr, from Ibn Abi Mulaykah, from Ya‘la ibn Mamlak, from Umm ad-Darda’, from Abu ad-Darda’, from the Prophet (peace and blessings of Allah be upon him), who said: "The heaviest thing in the scale on the Day of Resurrection is good character."