From Sahih Muslim (Hadith 1897):
Amir b. Wathila reported that Nafi' b. 'Abd al-Harith met 'Umar at 'Usfan and 'Umar had employed him as collector in Mecca. He (Hadrat 'Umar) said to him (Nafi'):Whom have you appointed as collector over the people of the valley? He said: Ibn Abza. He said: Who is Ibn Abza? He said: He is one of our freed slaves. He (Hadrat 'Umar) said: So you have appointed a freed slave over them. He said: He is well versed In the Book of Allah. the Exalted and Great, and he is well versed In the commandments and injunctions (of the Shari'ah). 'Umar said: So the Prophet (ﷺ) said: By this Book, Allah would exalt some peoples and degrade others
Reference: Muslim 1897