An-Naḥl: 89
{A witness over them from among themselves}
Meaning: their Prophet. For the prophets of the nations were sent from among them, and from among themselves.
{And We brought you}
O Muhammad.
{As a witness over these}
Over your nation.
{A clarification (tibyān)}
An eloquent explanation. The form tibyān is analogous to tilqāʾ in the kasra (vowel 'i') of its first letter, though al-Zajjāj permitted its fathah (vowel 'a') outside of the Quran.
If you ask: How is the Quran a clarification {of everything}?
I say: The meaning is that it clarifies everything pertaining to religious matters. It does so by explicitly stating some things, and by referring others to the Sunnah—as it commands the following of the Messenger of Allah (peace be upon him) and obedience to him. It is said: "And he does not speak out of desire." It also encourages consensus (ijmāʿ) in His saying: {And follows other than the way of the believers} (An-Nisāʾ: 115).
The Messenger of Allah (peace be upon him) was pleased for his nation to follow his companions and emulate their footsteps, as he said: "My companions are like the stars; whichever of them you follow, you will be guided." They exercised ijtihād, used analogy (qiyās), and paved the paths of analogy and legal reasoning. Thus, the Sunnah, consensus, analogy, and ijtihād are all supported by the clarification of the Book. Hence, it is a clarification of everything.
An-Naḥl: 90
{Indeed, Allah orders justice and good conduct and giving to relatives and forbids immorality and bad conduct and oppression. He admonishes you that perhaps you will be reminded.}