What is the ruling on praying behind the extremists of the Shi'ah?

Question:
I once prayed in one of their mosques (the extremists of the Shi'ah) without knowing that they were of that group. Is my prayer valid?

Answer:
His prayer is not valid if the imam of the mosque is one of them; it is not valid. He must repeat it. Praying behind a disbeliever is not valid; it is only valid behind an open sinner. As for a disbeliever, no. They call upon the People of the House, seek aid from 'Ali, seek aid from Fatimah, seek aid from al-Hasan and al-Husayn, and they claim that they know the unseen. They curse the Companions. They are among the imams of disbelief.
Q: He also says: What is the ruling on their prayer where they combine the midday prayer with the afternoon prayer, and the sunset prayer with the night prayer?
The Shaykh: This is among their religious innovations. This is among their misguidance. One does not combine the midday prayer with the afternoon prayer, nor the sunset prayer with the night prayer, except for a legitimate religious excuse such as illness or travel. As for combining them without an excuse: it is invalid and not permissible.