Some people online, epesically the anti-Muslim brigade have been using some older documents claiming they are between the last Sassanid emperor Yazdegerd III and Arab Caliph Omar.
The letters are said to contain Omar's threat to Persians to convert to Islam and Yazdegerd replying insultingly while telling the Arabs to go back. Needless to say they have used dubious documents to claim they are true.
The Arabic letter has been claimed to have been translated in a video by a woman. It shows a fragment in Arabic, however some may notice this fragment only has a few Arabic words:
The claims about the document are actually false. It's from University of Pennsylvania and is about a watering hole drying up. You can still find its actual translation and link to high quality image here. Please scroll to section 12.06 near the bottom of article: http://ccat.sas.upenn.edu/rak/cajs/info/inventory.html
Here is the actual translation
1. [??] the watering spot Khamal has dried up. So know firstly
2. [veil; contract] considered the matter of crossing the river [???]
3. [...] Auspicious to us [since][the camels] were in a state [of disrepair]
4. "Help to the one who might go astray"
5. ...
Now concerning the translation of letter by Yazdegerd, it has been claimed to have existed as far as 1980s and the letter is kept secretly in British Museum. But neither it nor the letter are in existence, as proven by a British Museum official Ursula-Sims Williams: https://sites.google.com/site/persianforgery/.
In addition, a purpoted docuemnt from am old video of a woman has been used to prove the letter is in existence.
However, it is never said how she got his hands on it when it's locked in the Museum. Not to mention, the so-called Persian letter is very blurry. If you looked at it closely, you can make out a few Arabic letters like jīm جـ
Sadly I cannot find the source for this letter due to its blurriness. But we already know not to trust them. This is too more than likely with a forged translation just like the Arabic one.
