فَإِن كُنتَ في شَكٍّ مِمّا أَنزَلنا إِلَيكَ فَاسأَلِ الَّذينَ يَقرَءونَ الكِتابَ مِن قَبلِكَ ۚ لَقَد جاءَكَ الحَقُّ مِن رَبِّكَ فَلا تَكونَنَّ مِنَ المُمتَرينَ

So if you are in doubt, [O Muhammad], about that which We have revealed to you, then ask those who have been reading the Scripture before you. The truth has certainly come to you from your Lord, so never be among the doubters.