Surah Al-An'aam, Verse 19
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
Translation — Sahih International
Say, "What thing is greatest in testimony?" Say, "Allah is witness between me and you. And this Qur'an was revealed to me that I may warn you thereby and whomever it reaches. Do you [truly] testify that with Allah 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]."
Source
Kur'an-ı Kerim, En'âm Suresi 6:19
https://kuran.diyanet.gov.tr/mushaf/kuran-meal-2/6-suresi-19-ayet