Mosab Hassan Yousef is a Palestinian who worked undercover for Israel's internal security service Shin Bet from 1997 to 2007.

Shin Bet considered him its most valuable source within the Hamas leadership: the information Yousef supplied prevented dozens of suicide attacks and assassinations of Israelis, exposed numerous Hamas cells, and assisted Israel in hunting down many militants, including his own father, a Hamas leader Sheikh Hassan Yousef. In March 2010, he published his autobiography titled Son of Hamas.

(from Wikipedia)