What must a worshiper do to correct his prayer when he forgets the number of prostrations?

If you doubt whether you performed one prostration or two, assume the lesser number and perform an additional prostration afterwards. Then, perform two prostrations before the salām (greeting of peace that concludes the prayer). This applies if you are not frequently afflicted by obsessive doubts. However, if doubts and obsessions become frequent, disregard them, and you are not required to perform the prostrations of forgetfulness (sujūd al-sahw). Devotion and supplication during prostration are not causes of doubt or obsession.