Purity: Tayammum Replacing Ghusl or Wudu
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 711:
If someone who cannot perform ghusl wants to perform an act for which ghusl is obligatory, he must perform tayammum in place of ghusl. If someone who cannot perform wuḍūʾ wants to perform an act for which wuḍūʾ is obligatory, he must perform tayammum in place of wuḍūʾ.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)