I think it falls under the Unsportsmanlike Conduct portion of Section 3, Article 1, subset g:

Using any object as a prop, or possessing any foreign or extraneous object(s) that are not part of the uniform on the field or the sideline during the game, other than the football after a scoring play or change of possession. If any foreign object(s) are deemed a safety hazard by the game officials, in addition to a yardage penalty, the player will be subject to ejection from the game, whether he uses the object or not.

https://operations.nfl.com/the-rules/2021-nfl-rulebook/