Purity: Uncertainty Between Jabirah Wudu and Tayammum

Practical Laws of Islam as per the teachings of Ayatullah Sistani

Ruling 342:

If someone does not know whether his duty is to perform tayammum or jabīrah wuḍūʾ, he must perform both of them.

-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)