The default ruling for a mosque is that it is an endowment (waqf) and the five daily prayers are performed in it

Question:
He says: Honorable Shaykh, may Allah grant you success, is it necessary for the land of a mosque to be an endowment (waqf) for it to be considered a mosque, and what is the evidence?

Answer:
It is not called a mosque unless it is an endowment (waqf). As for a place where prayer is performed temporarily, it is not called a mosque; it is called a prayer place (musalla) only. Without a doubt, it is not a mosque unless the five daily prayers are performed in it. Yes.