In Islamic tradition and history it is said that Muhammad sent a letter to Khosrow II, a Sasanid king who was titled king of the kings, inviting him to Islam.
As history is an interactional phenomenon, thus one way to validate a historical event is to hear it from the other party involved.
In this case, finding a footprint of such a letter in Persian's history would be a historical proof for its existence and essence.
But I can't find one. One reason is that I am not a professional historian. I am only a researcher, mostly doing interdisciplinary research.
Do we have any historical record in Persian's history (not Arabs, not Muslims) to mention this letter and its subject?