Bukhari

Explanation:
The reason is that no limb of ablution (wudu) should remain dry; otherwise, that very limb will be subjected to the punishment of Allah on the Day of Resurrection.

Hadith Authentication (Takhrij al-Hadith):
[140۔ البخاري فى : 4 كتاب الوضوء : 29 باب غسل الاعقاب 165، مسلم 242، ترمذي 41]

Linguistic Explanation:
«وَیْل» means destruction, punishment. «اَعْقَاب» is the plural of «عقب», which means heel, the back part of the foot.

Understanding the Hadith:
In one narration, it is mentioned that the Prophet sallallahu alayhi wa sallam saw a man with a spot on his foot, the size of a fingernail, that was dry. The Prophet sallallahu alayhi wa sallam said, "Go back and perform ablution (wudu) properly." [صحيح : صحيح أبوداود، أبوداود 173، ابن ماجه 665، احمد 146/3]

It is understood from this that it is obligatory to wash all the limbs of ablution (wudu) completely, and if even a small part of any limb remains dry, the ablution will not be complete.

Hadith Commentary:
The intent here is that no limb involved in ablution (wudu) should remain dry; otherwise, that very limb will be subjected to the punishment of Allah on the Day of Resurrection.

Hadith Commentary:
The mention of heels in the hadith has come because, at that time, the situation related to them was present; otherwise, what is meant is every limb of ablution (wudu) regarding which, generally, negligence or laziness is shown in washing it thoroughly—for example, the heels and the lower part of the feet, etc. Thus, there is further clarification of this in the hadith.

It is narrated from Abdullah bin Harith radi Allahu anhu that the Messenger of Allah sallallahu alayhi wa sallam said:
“There is a punishment of fire for dry heels and the soles of the feet.”
(Musnad Ahmad: 191/4)

Imam Bukhari rahimahullah has mentioned the practice of Ibn Sirin for this very purpose, because sometimes a ring is tight and water does not reach easily, so he would move it.
(Fath al-Bari: 350/1)

Similarly, generally, women use such items for makeup on their faces that the layered coating becomes a barrier to water reaching the limbs of ablution—for example: lipstick for the lips, layered powder for the face, and nail polish, etc. Therefore, women should avoid using such beautification items that become a barrier to water reaching the skin of the limbs of ablution.

However, during specific days, there is no harm in using such adornment items for their husbands.

Hadith Commentary: Benefits and Issues: The purpose of all the ahadith of Bilal is that washing the feet is obligatory; there is no alternative to washing them. Wiping (masah) over the feet without leather socks (khuffayn) or socks (jurabayn) is not sufficient, and there is no need to wipe in addition to washing. The opinion of those who consider wiping over the feet to be obligatory, or who give a choice between washing and wiping, or who consider both to be obligatory, is not correct. Furthermore, the feet must be washed completely; since there is a possibility that the heels may remain dry, special emphasis has been placed on washing them.

Hadith Commentary:
Vocabulary of the Hadith:

(1)
مَطْهَرَةٌ (mat-harah):
If the letter "meem" carries a kasrah (i.e., is pronounced "mit-harah"), the meaning will be: an instrument for ablution (wudu)—that is, any vessel from which ablution is performed.
And if the "meem" carries a fathah (i.e., is pronounced "mat-harah"), the meaning will be: a place for washing.
Here, taking the meaning as "vessel" is more appropriate.

(2)
عَرَاقِيب (a'raqib):
This is the plural of "عرقوب" (urqub),
the tendon above the heel.

1⎋:
This hadith indicates that if there are no leather socks (khuffayn) or socks (jurab) on the feet, then washing them is obligatory; wiping (masah) is not sufficient, as is the view of the Shia. For if the obligation could be fulfilled by wiping, the Prophet (sallallahu alayhi wa sallam) would not have said: “Woe to the heels and soles of the feet from the Fire.”