Quran chapter 10 surah 10.  

If you are in doubt about what God has revealed to you in the Quran then question the Jews and the Christians who have been reading the Book before you (Quran 10:94) meaning the Quran is correctly understood when it agrees with the Torah Taurat which is with the Jews and the Gospel Injil which is with the Christians as proof God is real because the proof God is real is that no one can change the word of God.