Did Benjamin Franklin really call the prophet Muhammad a "model of compion"?

The other day, on the August 5th Daily Show, Akbar Ahmed was being interviewed and he claimed Benjamin Franklin called Mohammed a "model of compion". This caught be off guard and didn't sound very Franklinish. I did a search and found this quote on several websites, but I have been unable to locate a primary source. I've tried to find evidence for this, but at this point I'm uming this is lie concocted as part of Muslim propaganda campaign to make their religion appear peaceful. Can anyone prove me wrong?