During a middle school game today, the umpire called the third base runner out for not tagging up after each pitch. Can someone tell me what rule applies to this or how to research it. Thank you.

Ther isn't any such rule.

was it maybe after a foul ball? even so, he shouldn't have called the runner out since you shouldn't put the ball back in play until the runners have retouched after a foul ball.

This is covered in the "Smitty's Baseball Rules, Annotated. Check out section 7.12 between "The hands are part of the bat" and "home plate is foul." It clearly states that every runner must tag their base after every pitch AND when time is called.

If you can't find a copy of this in print, just ask any Little League parent or any umpire you see brushing off second base or wearing his ball bags on the bases. They'll be happy to explain it to you.

I did my first small field games of the year today, 3 games. In the second game I have a batter who is crowding the plate and I hear from the stands, "he is out of the box blue, my God, his hands are over the plate". Pitcher was in his windup and not delivered yet. I just smiled under my face mask...

I do this every game for every runner. Sure does shorten the games up. Dlb header in 1-1/2 Hrs last night.
Coaches argued the first time, threw them out and law and order prevailed. Thanks for the advice Luke.