Is a person suffering from obsessive doubts (al-musawis) obligated to perform an act if he is unsure whether he is in an obligatory prayer (fard) or a supererogatory one (sunnah)?

One afflicted by obsessive doubts should disregard these doubts and rely on the certainty that preceded the doubt. So, whoever doubts whether they are praying an obligatory prayer or a supererogatory one, let them abandon the obsessive doubt and assume they are performing the obligatory prayer. Thus, the cure for obsessive doubts is to disregard them and pay no attention to them.