Does the prayer of one afflicted by obsessive doubts become invalid if he is assailed by the intention to change the prayer or feels that he has exited from it?

Satan is keen on corrupting prayer with whispers (waswasah). The Prophet, peace and blessings be upon him, clarified the remedy for this, which is seeking refuge in Allah from the accursed Satan and spitting lightly to the left three times when one feels the whispers of a devil called "Khanzab." The questioner is advised to do what the Prophet, peace and blessings be upon him, commanded, and not to pay attention to the whispers, while increasing supplication, recitation [of the Quran], and the morning and evening remembrances (adhkar).