– As for the arms: Allah set their limit “to the elbows.” The scholars have explained that the preposition “to” (ilā) here means “up to and including the elbows,” and they supported this by noting that the Prophet ﷺ caused the water to flow over his elbows. The same construction applies to the feet in His words, “to the ankles.”
– As for the head: it must be wiped in its entirety, for Allah commanded that it be wiped, and the preposition bi- (“with”) denotes direct contact, requiring that the wiping adhere to the whole area wiped; it is not a particle that indicates wiping only part of it.
– Among the rulings derived: maintaining the prescribed order among these four limbs is a condition. Allah arranged them in sequence and inserted a limb that is wiped between limbs that are washed; no benefit for this has been identified other than enforcing the order. The same is affirmed by the general meaning of the Prophet’s statement, “I begin with what Allah began with” (1). Though originally spoken concerning the rites of Hajj, it applies to all matters, and every narrator who described the Prophet’s ablution ﷺ mentioned it in that precise order.
– Another ruling: continuity (muwālāt) is also a condition. Allah mentioned the limbs of ablution joined by the conjunction wa, which indicates that this act of worship is to be completed in a single span of time; if one separates them into two different times, it ceases to be one act of worship—just as splitting a single prayer would. Furthermore, the Prophet’s constant practice—so vivid that you can almost see him—was to perform the limbs of his ablution consecutively. This proof is stronger than the line of argument many jurists take from the story of “the man with the patch of dry skin,” whom the Prophet ordered to repeat his entire ablution. Although that report does carry some indication for this question, it is possible that the Prophet’s order in that case was like his command to the man who prayed badly to repeat his prayer: he had seen him performing his ablution deficiently and failing to complete it.