THE LOST ARK OF THE COVENANT, by Tudor Parfitt, is excerpted by MSNBC. Here's an excerpt of the excerpt:

This was the gist of the document he held in his hand: Muhammad swears in the letter that it was the Jews of Medina and the other oasis towns of Arabia who had always come to his aid in his many battles against the heathen tribes of the desert. The Jews were even ready to desecrate their holy Sabbath to help him. They never left his side. They never betrayed him. During a single bloody campaign, the Jews killed over 20,000 heathen enemies of the Prophet: 7,000 knights, 7,000 regular horsemen, and 7,000 foot soldiers.

“This is what the Prophet actually promised the Jews,” declared Reuven reverently, raising one finger for emphasis. “Not centuries of contempt and persecution!” “Just listen.” He put on a pair of reading glasses, scrutinized the document, and read aloud. “‘O men of the Children of the lost ark of the covenant Israel, by Allah I shall reward you for this … I shall grant you my protection, my covenant, my oath and my witness for as long as I live and as long as my community shall live after me, until they see my face upon the Day of Resurrection.’ “Did you hear that?” he asked, his voice suddenly shrill, thrusting the document in my face and revealing an immaculately laundered cuff. “If the Muslim world knew about this, they would change their attitude to Israel overnight! There’d be no more Arab–Israel wars! No more terrorist attacks!”

Unfortunately, there was more to the letter than met the eye. It was probably quite old, I could see that.

The body of the text was in Arabic and there was a short introduction in Hebrew.

I knew something about Hebrew palaeography—the study of the form of ancient writing—and I could see this was a medieval Hebrew Yemenite script.

This much was genuine. Then I recalled that once in the Yemen I had seen an almost identical document in the home of an antiquarian in Sana’a, the capital of the Yemen. It was called Dhimmat al-Nabi (The Protection of the Prophet) and was an ancient Jewish fabrication, an old forgery, which the Yemenite Jews had created to counter the animosity of their Muslim neighbors. There was no Jewish community in the Muslim world quite as wretched and persecuted as the Jews of the Yemen. They needed all the help they could get. However, this document would not persuade many Muslim scholars to turn their received opinions upside down. It would not change the world. “It’s a shame,” I said, “but it is a forgery. A very old forgery.” A yellow hamseen wind was blowing in from the desert. It was stiflingly hot. Reuven’s face fell when I gave him my assessment of his document, and he grew silent. He just sat there grimacing, rubbing the side of his head where he had been grazed by an Egyptian bullet in the last of his wars.

Background here.