Verse 13

أَمْ يَقُولُونَ ٱفْتَرَىٰهُ قُلْ فَأْتُوا۟ بِعَشْرِ سُوَرٍ مِّثْلِهِۦ مُفْتَرَيَـٰتٍ وَٱدْعُوا۟ مَنِ ٱسْتَطَعْتُم مِّن دُونِ ٱللَّهِ إِن كُنتُمْ صَـٰدِقِينَ

Or they may say he fabricated it. Say, then bring ten suras like it, fabricated, and call upon whomever you can besides God if you are truthful.