What is the ruling on praying in a mosque built on land usurped by the Israeli government, given that those responsible for its construction refused to use adjacent donated land?

The land must have a legitimate owner. If the occupying government seized it from its owner, or if Muslims took it from the occupying government by force, then the original owner, if known, must be approached and appeased by offering the land's price or a substitute for it. If the owner does not agree, then he has a greater right to his land, and the buildings and plants must be removed, or their value guaranteed to the owner.

As for praying on usurped land, the Hanbalis consider the prayer invalid, while the majority of scholars hold it to be valid, though sinful. This latter view is the more correct one, because the prohibition is on usurpation itself, not on praying in a usurped place.