Musnad Ahmad 1301:14

'Abd al-Malik ibn 'Amr and Ibn Abi Bukayr narrated to us, they said: Ibrahim—meaning Ibn Nafi'—narrated to us, from al-Hasan ibn Muslim, from his maternal uncle 'Ata' ibn Nafi', that they entered upon Umm al-Darda', and she informed them that she heard Abu al-Darda' say: The Messenger of Allah (may Allah bless him and grant him peace) said: "Indeed, the best thing in the scale"—Ibn Abi Bukayr said: "the heaviest thing in the scale"—on the Day of Resurrection is good character."