Purity: Immersive Ghusl during Ihram
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 370:
A person in the state of iḥrām[55] 1for hajj or ʿumrah must not perform immersive ghusl. However, if he forgetfully performs immersive ghusl, his ghusl is valid.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)
1
Iḥrām here refers to the state of ritual consecration of pilgrims during hajj andʿumrah.