Is a father's obedience obligatory concerning prayer in a mosque known for heresies and for attacking Salafis, especially since he is ill and dislikes the people of Salafism, and might become angry if he is not obeyed?

A father has a great right to dutifulness and kindness. Therefore, one must bring joy to him and avoid angering him. Regarding the issues mentioned: Clasping the hands in prayer is a Sunnah (definition), not an obligation, and it can be omitted to please the father. It is permissible to pray behind an Imam unless his creed involves polytheism (shirk (definition)) or apostatizing innovation (bid'ah mukaffirah). As for innovations that do not lead to apostasy, such as collective supplication (du'a jamā'ī), they do not prevent praying behind him. Congregational prayer in the mosque is obligatory, and it is not permissible to obey one's father in abandoning it. If praying behind a specific Imam is not valid, one can pray behind another Imam, even if it is a second congregation, while continuing to be dutiful to the father. Adherence to religion may be accompanied by trials. One must remain steadfast in truth and invite to the Salafi creed, even if this angers one's family. For whoever pleases Allah, He will suffice him against the troubles of people.