Musnad Ahmad 30:2936

'Abd al-Rahman narrated to us, he said: Hammad ibn Salamah narrated to us, from Muhammad ibn Ziyad, he said: I heard Abu Hurairah saying: I heard Abu al-Qasim (may Allah bless him and grant him peace) saying: "The best of you in Islam are the best of you in character, when they attain understanding [of the religion]."