What is the ruling on praying behind someone who shows off in their prayer (I mean when he leads us in prayer)?

Hypocrisy (Riyaa') is a matter of the heart, and the congregant (ma'mum) cannot ascertain its presence in the heart of his Imam. Therefore, the default is the validity of the Imam's prayer. Even if the Imam were a hypocrite, there is a detailed explanation regarding the invalidation of his prayer, and the invalidation of the Imam's prayer does not always necessitate the invalidation of the congregant's prayer. There is no blame on the congregant because he cannot ascertain the existence of that which invalidates the prayer.