Purity: Defective Immersive Ghusl
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 368:
If after performing immersive ghusl, one realises that water has not reached all the parts of his body, he must perform ghusl again, irrespective of whether or not he knows which parts of his body the water did not reach.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)