Sahih Muslim Book 4, Hadith Number 1780.

Chapter : Excellence of the recitation of the two suras concerning the taking of refuge.

‘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 Holy Prophet (may peace be upon him) said: By this Book, Allah would exalt some peoples and degrade others.