Purity: Doubts After Tayammum
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 702:
If one doubts whether or not he has correctly performed tayammum after wiping the left hand, his tayammum is valid. In the event that one’s doubt is about the wiping of the left hand, it is necessary for him to wipe it unless it can commonly be said that he has finished performing tayammum; for example, he has started to perform an act that requires purification [such as prayers], or the close succession (muwālāh) [in performing tayammum] has not been maintained.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)