Here’s his joke:

Nasser chose Sadat as his Vice President because he was so much dumber than Nasser that he did not believe he would use his position to overthrow him. When Sadat died, Nasser chose Mubarak to be his vice president for the same reason. When Nasser died Mubarak could not find anyone dumber then himself so he did not appoint a vice president.

Actually there’s some truth to Letterman’s joke, Mubarak had been grooming his son Gamal to succeed him, but that’s not going to happen.