Purity: Intention (Niyyah) in Tayammum
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 694:
When making the intention to perform tayammum, it is not necessary for one to specify whether the tayammum is in place of ghusl or wuḍūʾ. However, in cases where two tayammums must be performed, it is necessary to specify each of them [in one’s intention]. In the event that one tayammum is obligatory for a person and he makes the intention that he is performing his current duty, then even if he makes a mistake in determining his duty, his tayammum is valid.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)