"Say, what thing is greatest in testimony?" Al-Kalbi said:

The polytheists among the people of Mecca said to the Prophet: Who knows that you are the Messenger of Allah so that they may bear witness for you? So Allah revealed: "Say, Allah is witness between me and you." He is the witness that I am His Messenger.

"And this Quran was revealed to me that I may warn you thereby and whomever it reaches," meaning: whoever the Quran reaches.