Does the prayer become invalid, necessitating its repetition, if the worshiper stands for the third rak'ah during the first tashahhud, after saying "and I bear witness that Muhammad is the Messenger of Allah," concurrently with the takbir for standing?

The questioner is afflicted with waswasah (whispers/obsessive thoughts) during prayer. We advise her to fear Allah the Almighty and to stay away from waswasah and not pay attention to it. Her prayer is not invalidated by what was mentioned, as the first tashahhud and sitting for it are considered sunnah according to most jurists.