Surah Al-An'am 6:19

قُلْ أَىُّ شَىْءٍ أَكْبَرُ شَهَـٰدَةً ۖ قُلِ ٱللَّهُ ۖ شَهِيدٌۢ بَيْنِى وَبَيْنَكُمْ ۚ وَأُوحِىَ إِلَىَّ هَـٰذَا ٱلْقُرْءَانُ لِأُنذِرَكُم بِهِۦ وَمَنۢ بَلَغَ ۚ أَئِنَّكُمْ لَتَشْهَدُونَ أَنَّ مَعَ ٱللَّهِ ءَالِهَةً أُخْرَىٰ ۚ قُل لَّآ أَشْهَدُ ۚ قُلْ إِنَّمَا هُوَ إِلَـٰهٌ وَٰحِدٌ وَإِنَّنِى بَرِىٓءٌ مِّمَّا تُشْرِكُونَ

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. 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

Tafsir

When they said to the Prophet s ‘Bring us someone to testify to the truth of your prophethood for the People of the Scripture have denied you’ the following was revealed Say to them ‘What thing is greatest in testimony?’ shahādatan this is for specification and is derived from the implied subject of the sentence. …

Excerpt from Tafsir al-Jalalayn, commentary on 6:19, via Quran.com. The reader has the full commentary and 9 more tafsirs.