16:89
One day We shall raise from all Peoples a witness against them, from amongst themselves: and We shall bring thee as a witness against these (thy people): and We have sent down to thee the Book explaining all things, a Guide, a Mercy, and Glad Tidings to Muslims.
The Qurʾān describes itself as a comprehensive source of guidance. In the verse stating that the Book was revealed “as an explanation of everything,” classical commentators have understood this to mean that the Qurʾān contains the foundations of all beneficial knowledge relevant to human life. Ibn Masʿūd is reported to have explained that the Qurʾān clarifies matters pertaining to both past and future events, as well as distinguishing what is lawful and unlawful. It also provides guidance concerning matters of faith, worldly conduct, livelihood, and the ultimate destiny of human beings in the Hereafter. In this sense, the Qurʾān functions as guidance for the hearts, a source of mercy, and glad tidings for believers.
Some scholars, such as al-Awzāʿī, interpreted the phrase “an explanation of everything” as referring to the Qurʾān in conjunction with the Sunnah, which further clarifies and explains its teachings. The statement that the Book was revealed to the Prophet Muhammad is also connected to the idea that he would serve as a witness regarding the message he conveyed. According to this interpretation, the One who entrusted the Prophet with the responsibility of delivering the revelation would also question him about the fulfillment of that responsibility on the Day of Resurrection.
This concept is reinforced by other Qurʾānic passages that indicate both messengers and their communities will be questioned regarding their response to divine guidance (Qurʾān 7:6; 15:92–93). Another verse describes the Day when God will gather the messengers and ask them about the response they received to their message (Qurʾān 5:109). Similarly, the verse stating that the One who obligated the Qurʾān upon the Prophet will bring him back to the final return (Qurʾān 28:85) has been interpreted as referring to the Day of Resurrection, when the Prophet will be returned to God and asked about the mission entrusted to him. This interpretation is regarded by some commentators as a coherent explanation linking the revelation of the Qurʾān with the accountability associated with conveying it.
References:
Kathir, Ibn. Tafsir Ibn Kathir. (Surah 16 verses 89).