Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.028A.

Allah mentioned the obligations in a specific order. He also differentiated the legs from the hands–though both of them have to be washed–from the head, which only needs to be wiped. The polytheists of Arabia would not differentiate items unless there was some benefit in doing so. The way Allah structured the ablution made it easier for them to comprehend it. Al-Ma’idah 6 explains what is obligatory and it falls under the generality of the Prophet’s statement, “Begin with what Allah began with.” The Prophet used to follow that sequence as one of ablution’s principles. There is no such report that the Prophet, upon whom be peace, ever departed from that sequence. Ablution is part of worship, and in matters of worship there is no room for anything except doing what has been commanded.