Does the prayer of one who erred in pronouncing "Rasulih" (His Messenger) in the first Tashahhud become void, by sometimes pronouncing the "seen" as a "saad" and not correcting it?

The first Tashahhud is a Sunnah. As for the last Tashahhud, a linguistic error in it may invalidate the prayer if it changes the meaning. However, some scholars have been lenient regarding similar-sounding letters. A person afflicted with Waswasah (obsessive doubt) should not pay attention to her doubts regarding the pronunciation of the Tashahhud, and her prayer is valid. If she becomes certain of an error, she should correct it as long as she is still in the prayer. It is permissible for her to adopt the view of those who do not invalidate the prayer due to such an error, because a person with Waswasah is allowed to follow the easiest opinions. The cure for Waswasah is to disregard it.