A party desiring to recuse a judge of a district court shall file a written motion therefor assigning the ground for recusation. This motion shall be filed prior to trial or hearing unless the party discovers the facts constituting the ground for recusation thereafter, in which event it shall be filed immediately after these facts are discovered, but prior to judgment. If a valid ground for recusation is set forth in the motion, the judge shall either recuse himself, or refer the motion to another judge or a judge ad hoc, as provided in Articles 155 and 156, for a hearing.