Purity: Doubts During Tayammum
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 701:
If while performing tayammum one doubts whether or not he has forgotten a certain part of it, in the event that he has passed that stage [i.e. he has performed that particular part of tayammum], he must not heed his doubt; and if he has not passed that stage, he must perform that stage.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)