The Quran

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

Muhammad Asad

and so they assert, "[Muhammad himself] has in vented this [Qur'an]!"¹ Say [unto them]: "Produce, then, ten surahs of similar merit, invented [by yourselves], and [to this end] call to your aid whomever you can, other than, God, if what you say is true!²

For my rendering of the particle am at the beginning of this sentence as "and", see surah 10, note 61.
I.e., that a divine writ like the Qur'an could have been "invented" by a human being. Cf. 2:23, 10:37-38 and 17:88, as well as the corresponding notes.