Is it permissible to perform prayers and Friday prayer in a mosque, most of whose cost (more than 70%) was built with ill-gotten money acquired from drugs, alcohol, gambling, and fraudulent social welfare claims? And is it permissible to pray at home instead, given that the people of this mosque love innovations and hate the Sunnah?

It is permissible to build mosques and pray in them with illicit money whose owner is unknown or to whom it cannot be returned. Ibn Rushd al-Jadd was of the opinion that illicit money whose origin is unknown is treated as fay' (spoils of war distributed among Muslims) and not as sadaqah (charity). As for praying behind an innovator imam, it is permissible to pray behind him by agreement of the Sunnis if no other imam is available for certain types of prayers, such as Friday prayers and the two Eids. If there is another non-innovator imam, then praying behind him is better and more preferable. Praying behind an innovator is valid according to Al-Shafi'i and Abu Hanifa, and it is valid with dislike (karaha) according to Malik and Ahmad.