What is the ruling on someone who made a mistake in the final Tashahhud and said: "Astaghfirullah (I seek forgiveness from Allah)" instead of "As-salamu alaykum wa rahmatullah (Peace and mercy of Allah be upon you)"; is their prayer valid?

The Tasleem (salutation of peace) is one of the pillars of prayer. Therefore, if one forgets it and replaces it with something else, that is not sufficient; rather, one must perform the Tasleem. The prayer is valid if one performs it soon after remembering. However, if a long time passes, or if one does not perform it, then the prayer is invalid, and it must be repeated.