Purity: Forgetfulness and Usurped Items in Tayammum
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 685:
If a person forgetfully or neglectfully performs tayammum with a usurped thing, his tayammum is valid. However, if he has usurped that thing and he forgets that he has usurped it, then based on obligatory precaution, his tayammum is invalid.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)