the ruling for praying on usurped land.

It is legally invalid for anyone to usurp the property of another and build a mosque on it; Allah is good and accepts only what is good.
Allah says in His Noble Book, al-Qur'an,

"[Oh Prophet,] never pray in that mosque. You should rather pray in a mosque founded from its first day on consciousness of God: in this mosque there are men who desire to grow in purity—God loves those who seek to purify themselves." [Qur`an 9:108]

It is legally impermissible to usurp land as attested to by the hadith narrated by 'A`isha, may Allah be pleased with her, "Whosoever takes even one hand span of land unjustly, on the day of Resurrection, he will be bridled with the weight of the seven earths" [Bukhari and Muslim].

Scholars have stated that a place becomes a mosque by praying in it or when the owner of the place says, "I establish it as a mosque." There is no doubt that ownership of the place must be valid at the time the mosque is established since in this instance it becomes an endowment.

Scholars agree that among the conditions of a valid endowment is that the person making the endowment be in possession of the property at the time he establishes the endowment. As for praying in such a mosque, al-Nawawi said in Al-Majmu' Sharh al-Madhhab (3/164): "There is a scholarly consensus that praying on usurped land is prohibited and that scholarly difference [on this matter] concerns the validity of [prayer] performed on usurped land and its reward."

Al-'Ezz ibn 'Abd al-Salam wrote in Qawa'id al-Ahkam that if a person makes a spiritual retreat in mosque and finds out that it is built on usurped land, his retreat is nullified.

Allah Almighty knows best.