What should a worshiper do if he doubts not having recited "I bear witness that there is no god but Allah, and I bear witness that Muhammad is His Messenger" during the final Tashahhud, or before the Taslim in another prayer?

If a worshiper doubts a part of the prayer before the salām, he must return to complete what he doubted and what follows it, and no prostration of forgetfulness (sujud sahv) is required of him. As for doubt after finishing the prayer, it is not taken into consideration. And if a worshiper forgets the final tashahhud—which is a pillar according to some scholars—and remembers it a short time after the salām, he must return, perform it, and then prostrate. However, if a long period passes between the salām and remembering, the prayer becomes invalidated and he must repeat it.