http://Naksibendi.us/ * St Catherine's monastery has this to say about the Holy Prophet Muhammed (peace be upon him)'s letter:
According to the tradition preserved at Sinai, Mohammed (AS) both knew and visited the monastery and the Sinai fathers. The Koran makes mention of the Sinai holy sites. In the second year of the Hegira, corresponding to AD 626, a delegation from Sinai requested a letter of protection from Mohammed (S). This was granted, and authorized by him when he placed his hand upon the document. In AD 1517, Sultan Selim I confirmed the monastery's prerogatives, but took the original letter of protection for safekeeping to the royal treasury in Constantinople. At the same time, he gave the monastery certified copies of this document, each depicting the hand print of Mohammed (S) in token of his having touched the original. --- Greek Halki School of Theology, the only Greek Orthodox seminary in Turkey.