What is the ruling on praying behind an imam who moves a lot during prayer, but it is not known whether his movements reach the extent of invalidating the prayer?

The Prophet, peace and blessings be upon him, commanded stillness in prayer and forbade movement during it, because movement contradicts khushu' (devotional humility). Slight or infrequent movement does not invalidate the prayer. However, abundant, continuous movement for no legitimate Shar'i need does invalidate the prayer. The criterion for "abundant" here is what people commonly understand, such that one would think that the person performing it is not in prayer. Abundant but infrequent movement does not invalidate the prayer.