Incomplete pass reversed to touchdown

[starbury]

We were watching an NFL playoff game and there was a pass to end zone which was initially ruled incomlete. Right away I yelled 'challenge it'. However my friend started arguing that any incomplete pass could be challenged and overturned but not in the end zone, not for a touchdown. This really surprised me because he knows all about football but apparently not this rule. We went on arguing about this one and finally had a bet for 12-pack of beer.

Do you know where I could find an official NFL rule book on the internet??? I tried but couldn't find it on NFL.com. I'm pretty sure about this one having seen it happen a few times ...for example in Super Bowl a couple of years back with Bucs and Raiders. Joey Porter caught a long pass which was first ruled incomplete but reversed for a toucdown.