http://en.wikipedia.org/wiki/Barbary_pirates
In 1786, Thomas Jefferson, then the ambassador to France, and John Adams, then the ambassador to Britain, met in London with Sidi Haji Abdul Rahman Adja, the ambassador to Britain from Tripoli. The Americans asked Adja why his government was hostile to American ships, even though there had been no provocation. The ambassador's response was reported to the Continental Congress:
It was written in their Koran, that all nations which had not acknowledged the Prophet were sinners, whom it was the right and duty of the faithful to plunder and enslave; and that every mussulman who was slain in this warfare was sure to go to paradise. He said, also, that the man who was the first to board a vessel had one slave over and above his share, and that when they sprang to the deck of an enemy's ship, every sailor held a dagger in each hand and a third in his mouth; which usually struck such terror into the foe that they cried out for quarter at once. [14]
As Wikpedia is often suspect, it would be of great service here if you could reference the congressional record or the Library of Congress for the primary source documentation of the quote. I have heard or read of this quote several times previous, but have not had success in finding the primary source.