Musnad Ahmad 2:151

Abu Kamil narrated to us, Ibrahim bin Sa’d narrated to us, Ibn Shihab narrated to us, (and) Abd al-Razzaq narrated to us, Ma’mar informed us, from al-Zuhri—the meaning—from Abu al-Tufayl ‘Amir bin Wathilah: That Nafi’ bin ‘Abd al-Harith met ‘Umar bin al-Khattab at ‘Usfan, and ‘Umar had appointed him over Makkah. ‘Umar said to him, "Whom did you appoint as a successor over the people of the valley?" He said, "I appointed Ibn Abza over them." He said, "And who is Ibn Abza?" He said, "A man from our freed slaves (mawali)." ‘Umar said, "You appointed a freed slave over them?" He said, "He is a reciter of the Book of Allah, knowledgeable of the laws of inheritance, and a judge." ‘Umar said, "Verily, your Prophet, may Allah bless him and grant him peace, said: 'Indeed, Allah raises some people by this Book, and lowers others by it.'"