Purity: Missed Areas Discovered After Ghusl
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 364:
If after performing ghusl one realises that he has not washed an area of his body, in the event that the unwashed area happens to be on the left side of his body, it is sufficient for him to wash only the unwashed area. If the unwashed area happens to be on the right side of one’s body, the recommended precaution is that after washing the unwashed area, he should rewash the entire left side. Furthermore, if the unwashed area happens to be on one’s head and neck, then based on obligatory precaution, he must rewash his body after he has washed the unwashed area.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)