"And the day We will raise among every nation a witness over them from themselves," meaning: their Prophet; he is a witness over them. "And We brought you, [O Muhammad], as a witness over these," meaning: his nation. "And We have sent down to you the Book as clarification for all things," meaning: what He has clarified therein of the lawful and the unlawful, and everything that Allah has revealed within it.

Surah An-Nahl, from verse (90) to verse (92).