(Qasas) 28 : 49
قُلْ فَأْتُوا بِكِتَابٍ مِنْ عِنْدِ اللَّهِ هُوَ أَهْدَىٰ مِنْهُمَا أَتَّبِعْهُ إِنْ كُنْتُمْ صَادِقِينَ
You say: "Then bring a Book from Lord Allah which is more guiding than either of Them, so that I may follow It if you are so truthful".
The Fujjar are reading in verses 21: 24-25 that all 313 Messengers have been sent down with the only Truth and Proof Adhikr. Adhikr revealed to Messenger Muhammad safeguards and testifies all previous 312 Books as explained in verse 5: 48. See explanation 6: 155-157 and 16: 89.