No rule for this one
No ending or word group predicts the article of Bein. Learn it as a pair and say it out loud: das Bein.
das Bein
It's das Bein. Bein is neuter, plural die Beine.
Mein Bein tut weh.
My leg hurts.
No ending or word group predicts the article of Bein. Learn it as a pair and say it out loud: das Bein.