A party desiring to recuse a trial judge shall file a written motion therefor assigning the ground for recusation. The motion shall be filed prior to commencement of the trial unless the party discovers the facts constituting the ground for recusation thereafter, in which event it shall be filed immediately after the facts are discovered, but prior to verdict or judgment. If a valid ground for recusation is set forth in the motion, the judge shall either recuse himself, or refer the motion for hearing to another judge or to a judge ad hoc, as provided in Article 675.