Illegal number and penalty question?

[JRutledge]

I normally can find this stuff quickly but for some reason I cannot.

There was a game recently that was brought to my attention where this took place.

A team had a number 7 and 8 and they played and then noticed. Is this a technical foul if they were playing with an illegal number? Or is this a situation where they just do not play with those illegal numbers? BTW, when noticed the players were removed from the game and did not play.

I need a rules reference.

Peace