Sheikh Fawzān – may Allāh preserve him - said ghusl is using water on the entire body in a particular manner as an act of worship. The evidence for it being obligatory is where Allāh says (the meaning of which translates as) “And if you are in a state of impurity, purify yourselves” [Al-Mā`idah, Āyah No. 6]. [Page 44 of Sheikh Fawzān’s Al Mulakhkhaṣ Al Fiqhī (Summary of Islamic Jurisprudence) (Arabic version)].