Purity: Ensuring Completeness in Ghusl
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 362:
In case a person is not certain that he has completely washed both parts – i.e. his head and neck, and his body – for him to be certain that he has washed both parts, when he washes one part, he must wash an area of the other part as well.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)