Observe the Congregational Prayer, even if it be behind One You dislike

Question :

I entered the mosque to pray, but I found an Imam whom I do not like to pray behind; what should I do so that I get the reward of the congregational prayer?

Answer:

If you entered the mosque to offer the congregational prayer and you found them praying, then pray with them, even if the Imam is a person whom you dislike, because the congregational prayer is obligatory and in such case you are bound by that duty. So it is not permissible for you to leave it. And there remains the question of why you dislike this man? Is it due to some defect in his religion, or personal enmity between you? If it is due to personal enmity, then it is incumbent upon the Muslim to remove whatever rancor exists between him and his brother and replace it with harmony and brotherly love, as Allah, the Most High says:

The believers are but brothers.

However, if your dislike of him due to some defect in his religion, then it is incumbent upon you to advise him and make clear to him what is wrong in his religion, so that he may correct it and to go straight upon the path which Allah has ordained. As for people renouncing each other whenever they see some defect in each other's religion, and contenting themselves with concealing their rancor and enmity for each other, this conflicts with the state of the believers about whom Allah says:

You (true believers in Islamic monotheism and real followers of Prophet Muhammad and his Sunnah) are the best of peoples ever raised up for mankind; you enjoin Al- Ma'ruf (i.e. Islamic monotheism and all that Islam has ordained) and forbid Al-Munkar (polytheism, disbelief and all that Islam has forbidden) and you believe in Allah.

Source:
Ibn 'Uthaimin
Fatawa Islamiyah, Vol. 2 Pages 393-394