What is the ruling on praying behind an Imam who sways his body right and left during prayer?

If the imam's movements in prayer become frequent and continuous, the prayer behind him becomes invalid, because excessive movement without necessity invalidates the prayer. However, if the movements are slight, the prayer is not invalidated, but they are disliked and reduce the reward. Scholars have differed on the criterion for what constitutes much and little movement. Some said it is determined by custom, while others said that excessive movement is that which makes an observer think that the doer is not in prayer.