The Imam عليه السلام clarified that if one delays washing the head until after washing the rest of the body, he is required to wash the head and then repeat the washing of the other parts of the body. This indicates that the order (tartib) in ghusl is obligatory. Had the order not been mandatory, there would have been no requirement to repeat the washing of the body. Furthermore, as previously mentioned, sufficient evidence has been provided for the obligation of maintaining order in both wudu and ghusl. This narration reinforces that obligation, and it is sufficient by Allah’s سبحانه وتعالى will.
Commentary #20