For some days I'm offering Salat at home because the Imaam is a blind follower of a particular Madhab. He renounced those who say Aameen loud at the end of Surah Faatihah, and raise hands before Ruku' and Sujood. Ironically enough he claimed these Sunnah acts as innovation while redeemed himself standing on the right path of Sunnah, though he is blindly following a Madhab. Now my question is whether denying an established Sunnah a major sin? If yes then will it be valid for us to offer prayer behind him? I have one more question, is it a must for a Muslim to offer Salat at mosque. Sometimes I feel tired and offer Salat at home, will it be amenable to major sin?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

In principle one should follow the Sunnah and the truth with evidence for whoever is able to study the evidences and decide which is the predominant ruling. However, it is permissible in general for Muslims (it might even be an obligation on them) to follow a given school, or to act according to the rulings of a trustworthy person. Therefore, if there is a religious reason for this man to follow a given school then one should not object to his act. But if he is able to understand the evidence, then one should advise him with good admonition and explain to him that it is an obligation to follow the evidence when it is proven authentic and correct. It can not be said that he committed a major great sin because, most likely, he is not deliberately denying the Sunnah. Nonetheless, it is not permissible to abandon praying behind him for this reason. The companions used to pray behind dissolute people, like the praying of Ibn 'Umar behind Al-Hajjaaj. It is permissible to pray behind a dissolute person and the benefit of unity amongst the Muslims has priority. Lastly, we bring to your attention that congregational prayer is an obligation according to the most preponderant opinion of the scholars so it is not permissible for any man who is able to pray in congregation not to attend congregational prayer without a sound valid excuse.

Allaah Knows best.