Musnad Ahmad 1302:41

Sufyan narrated to us, from ‘Amr, from Ibn Abi Mulaykah, from Ya‘la ibn Mamlak, from Umm ad-Darda’, from Abu ad-Darda’, attributing it to the Prophet: "Whoever is given his portion of gentleness has been given his portion of goodness, and there is nothing heavier in the scale than good character."