Sahih Muslim 817 a
'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.
وَحَدَّثَنِي زُهَيْرُ بْنُ حَرْبٍ، حَدَّثَنَا يَعْقُوبُ بْنُ إِبْرَاهِيمَ، حَدَّثَنِي أَبِي، عَنِ ابْنِ شِهَابٍ، عَنْ عَامِرِ بْنِ وَاثِلَةَ، أَنَّ نَافِعَ بْنَ عَبْدِ الْحَارِثِ، لَقِيَ عُمَرَ بِعُسْفَانَ وَكَانَ عُمَرُ يَسْتَعْمِلُهُ عَلَى مَكَّةَ فَقَالَ مَنِ اسْتَعْمَلْتَ عَلَى أَهْلِ الْوَادِي فَقَالَ ابْنَ أَبْزَى ‏.‏ قَالَ وَمَنِ ابْنُ أَبْزَى قَالَ مَوْلًى مِنْ مَوَالِينَا ‏.‏ قَالَ فَاسْتَخْلَفْتَ عَلَيْهِمْ مَوْلًى قَالَ إِنَّهُ قَارِئٌ لِكِتَابِ اللَّهِ عَزَّ وَجَلَّ وَإِنَّهُ عَالِمٌ بِالْفَرَائِضِ ‏.‏ قَالَ عُمَرُ أَمَا إِنَّ نَبِيَّكُمْ صلى الله عليه وسلم قَدْ قَالَ ‏
"‏ إِنَّ اللَّهَ يَرْفَعُ بِهَذَا الْكِتَابِ أَقْوَامًا وَيَضَعُ بِهِ آخَرِينَ ‏"‏ ‏.‏