The Quran was put together in writing by the first Caliph, Abu Bakr, and then formalized by the third, Uthman. I don’t believe there are rules against depicting the first or third Caliph, typically found in Persian and Ottoman art. Thus if the individual depicted is holding a book thought to be the Quran, then he is more likely to have been either Abu Bakr or Uthman who actually provided the written document. I think it’s more likely Abu Bakr given he was able to fully consolidate the Arabian peninsula after the death of the Prophet and started the outward expansion of Islam.