أَم يَقولونَ افتَراهُ ۖ قُل فَأتوا بِعَشرِ سُوَرٍ مِثلِهِ مُفتَرَياتٍ وَادعوا مَنِ استَطَعتُم مِن دونِ اللَّهِ إِن كُنتُم صادِقينَ

Or do they say, "He invented it"? Say, "Then bring ten surahs like it that have been invented and call upon [for assistance] whomever you can besides Allah, if you should be truthful."