أَوَلَمْ يَكْفِهِمْ أَنَّآ أَنزَلْنَا عَلَيْكَ ٱلْكِتَـٰبَ يُتْلَىٰ عَلَيْهِمْ ۚ إِنَّ فِى ذَٰلِكَ لَرَحْمَةً وَذِكْرَىٰ لِقَوْمٍ يُؤْمِنُونَ
Awa lam yakfihim annaaa anzalnaa 'alaikal kitaaba yutlaa 'alaikhim; inna fee zaalika larahmatanw wa zikraa liqawminy yu'minoon
Is it not enough for them that we have sent down to you the Book, recited to them? In this is surely mercy and a reminder for people who believe.
The answer to a demand for a sign points at something already in the room. Yutla is a passive in the present tense, so the recitation is described as continuous, which is exactly what a one-off wonder cannot be: a sign that happens once must afterwards be taken on report, while this one repeats whenever someone reads it aloud. Verse 48 had run the argument from the other end, resting on what the Prophet had never written.