Thank you all for your replies. The part of the rule book I was trying to find is the one where everybody quotes the possible 25k fine associated with it. After further investigation I found that there is something called the schedule of league fines which seems to be separate from the NFL official rule book. Yet I find no mention of it in there either.
I never knew it could be so hard to verify something that the media has been quoting.
PS: Now it seems it's from:
The
NFL's game operations manual states:
"Once the balls have left the locker room, no one, including players, equipment managers, ball boys, and coaches, is allowed to alter the footballs in any way. If any individual alters the footballs, or if a non-approved ball is used in the game, the person responsible and, if appropriate, the head coach or other club personnel will be subject to discipline, including but not limited to, a fine of $25,000."
Funny to me that there is no mention of the word tampering in this. Even though it means the same as above, some media like to substitute the buzz word tampering instead.
Found this article about the games operations manual.
“I will go in there before the game and say, 'How you doing? I just want to let you know I'm here today, and I'm keeping an eye on what you do. I'm not saying that you're doing anything wrong; I'm just saying I don't want to have to come down here.'"
MIKE KENSIL, VICE PRESIDENT OF GAME OPERATIONS
http://operations.nfl.com/football-ops/league-governance/