Qur'an 6:114

أَفَغَيْرَ ٱللَّهِ أَبْتَغِى حَكَمًا وَهُوَ ٱلَّذِىٓ أَنزَلَ إِلَيْكُمُ ٱلْكِتَـٰبَ مُفَصَّلًا ۚ وَٱلَّذِينَ ءَاتَيْنَـٰهُمُ ٱلْكِتَـٰبَ يَعْلَمُونَ أَنَّهُۥ مُنَزَّلٌ مِّن رَّبِّكَ بِٱلْحَقِّ ۖ فَلَا تَكُونَنَّ مِنَ ٱلْمُمْتَرِينَ

Shall I seek other than Allah for judge, when He it is Who hath revealed unto you (this) Scripture, fully explained? Those unto whom We gave the Scripture (aforetime) know that it is revealed from thy Lord in truth. So be not thou (O Muhammad) of the waverers.