"Is my prayer valid if I remember a minor impurity on my garment in the final Tashahhud, or must I interrupt it to change the garment? "

"Praise be to Allah, and prayers and peace be upon the Messenger of Allah and his family and companions. To proceed:

Prayer must be interrupted and repeated if impurity is found, because purity is a condition for the validity of the prayer. In the case of urination, scholars have differed: some see the prayer as invalidated absolutely, whether the impurity is small or large; the Hanafis consider forgivable only the amount that is equivalent to the size of a dirham; the Shafi'is excuse what the eye cannot perceive; and Shaykh al-Islam chose to forgive all minor impurities. The more precautious action is to make up for the prayer even if the urine is minor.

And Allah knows best."