Two weeks ago our Imam gave sermon against Salfis (ahle Hadees). Now I avoid praying behind him. Am I correct?

Praise be to Allah, the Lord of the World; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
As for your question about this Imam it is difficult to issue a Fatwa about him before we know exactly what he has said. Nevertheless, we think that if this Imam is among the innovators - which is very probable - it is not lawful to pray behind him as long as there is a Sunni Imam in the area. You should even try to remove him and get a Sunni Imam instead. If that is not possible or if it could cause more harm than benefits and if there is no Sunni Imam in the area, then you could pray behind him.
But if his innovation reaches the level of Kufr and if he refuses to give it up then it is not lawful to pray behind him.
Allah knows best.