Say, "What thing is greatest in testimony?" Say, "Allāh is witness between me and you. And this Qur’ān was revealed to me that I may warn you thereby and whomever it reaches.1 Do you [truly] testify that with Allāh there are other deities?" Say, "I will not testify [with you]." Say, "Indeed, He is but one God, and indeed, I am free of what you associate [with Him]."
Translation: Saheeh International (English)
1 At every time and place until the Day of Judgement.
Say, O Messenger, to the idolaters who deny you: Which thing is greatest and most honoured as a witness to my truthfulness? Say: Allah is the greatest and most honoured witness to my truthfulness. He is a witness between you and me. He knows what I have brought to you and how you respond. Allah has revealed this Qur’ān to me to warn you by it and to warn any human or jinn it reaches. You, O idolaters, believe that there are others worthy of worship together with Allah. Say, O Messenger: I do not bear witness to what you have acknowledged as it is false. Allah is only one God: He has no partner. I am free from all that you associate as partners with Him.