أَم يَقولونَ افتَراهُ ۖ قُل فَأتوا بِسورَةٍ مِثلِهِ وَادعوا مَنِ استَطَعتُم مِن دونِ اللَّهِ إِن كُنتُم صادِقينَ

Or do they say [about the Prophet], "He invented it?" Say, "Then bring forth a surah like it and call upon [for assistance] whomever you can besides Allah, if you should be truthful."