Is the prayer of one who says "Our Lord, and to You be the praise" during prostration out of forgetfulness valid, considering his suffering from obsessive-compulsive disorder? And is it permissible for him to follow other schools of thought to ease his obsessions? And what is the ruling on his past prayers that he performed with an incorrect ijtihaad (independent reasoning)?

The place for the saying "Rabbana wa lakal-hamd" (Our Lord, and to You be all praise) for the individual and the Imam is after rising from bowing (ruku'), while the congregant (ma'mum) says it during the rising. The questioner has erred by saying this dhikr during the descent for prostration (sujud (definition)), but this does not invalidate his prayer, and he may follow those jurists who permit it. He is not obliged to perform the prostration of forgetfulness (sujud as-sahw) according to some Hanbalis. A Muslim is required to learn and ask about what is unclear to him, and he must turn away from misgivings regarding the takbirat (saying "Allahu Akbar") and transitions (between prayer postures). He is not obliged to repeat prayers about which he had doubts. He may also follow schools other than the Hanbali school regarding the ruling on the takbirat of transition, as long as he does not intend to merely seek out concessions.