Musnad Ahmad 1302:20

Yahya ibn Sa'id narrated to us, from Shu'bah, who said: Al-Qasim ibn Abi Bazzah narrated to me, from 'Ata' al-Kaykharani, from Umm al-Darda', from Abu al-Darda', from the Prophet, peace and blessings of Allah be upon him, who said: "There is nothing heavier in the scale than a good character."