﴿ قُلْ أَىُّ شَىْءٍ أَكْبَرُ شَهَٰدَةًۭ ۖ قُلِ ٱللَّهُ ۖ شَهِيدٌۢ بَيْنِى وَبَيْنَكُمْ ۚ وَأُوحِىَ إِلَىَّ هَٰذَا ٱلْقُرْءَانُ لِأُنذِرَكُم بِهِۦ وَمَنۢ بَلَغَ ۚ أَئِنَّكُمْ لَتَشْهَدُونَ أَنَّ مَعَ ٱللَّهِ ءَالِهَةً أُخْرَىٰ ۚ قُل لَّآ أَشْهَدُ ۚ قُلْ إِنَّمَا هُوَ إِلَٰهٌۭ وَٰحِدٌۭ وَإِنَّنِى بَرِىٓءٌۭ مِّمَّا تُشْرِكُونَ ﴾

“Say: "What could most weightily bear witness to the truth?" Say: "God is witness between me and you; and this Qur’an has been revealed unto me so that on the strength thereof I might warn you and all whom it may reach." Could you in truth bear witness that there are other deities side by side with God? Say: "I bear no [such] witness!" Say: "He is the One God; and, behold, far be it from me to ascribe divinity, as, you do, to aught beside Him!"”

Please check ayah 6:21 for complete tafsir.