The obligatory parts of the ablution:
Ablution has certain components which, if not fulfilled according to the correct Islamic procedures, make one's ablution void.

Intention:
This is the desire to do the action and to please Allah by following His command. It is purely an act of the heart, for the tongue (verbal pronouncement, and so on) has nothing to do with it. To pronounce it is not part of the Islamic law. That the intention is obligatory is shown in the following: 'Umar related that the Prophet, upon whom be peace, said, "Every action is based on the intention (behind it), and everyone shall have what he intended..." (Related by "the group.")

Washing the face:
This involves "pouring" or "running" water from the top of the forehead to the bottom of the jaws, and from one ear to the other.

Washing the arms to the elbow:
The elbows must be washed, for the Prophet, upon whom be peace, did so.

Wiping the head:
This means to wipe one's head with his hand. It is not sufficient just to place the hand on the head or to touch the head with a wet finger. The apparent meaning of the Qur'anic words, "...and wipe over your heads..." does not imply that all of the head needs to be wiped. It has been recorded that the Prophet used to wipe his head three different ways:

 1. Wiping all of his head. 'Abdullah ibn Zaid reported that the Prophet, upon whom be peace, wiped his entire head with his hands. He started with the front of his head, then moved to the back, and then returned his hands to the front. (Related by "the group.").

 2. Wiping over the turban only. Said 'Amru ibn Umayyah, "I saw the Messenger of Allah, upon whom be peace, wipe over his turban and shoes." (Related by Ahmad, al-Bukhari and Ibn Majah). Bilal reported that the Prophet, upon whom be peace, said, "Wipe over your shoes and head covering." (Related by Ahmad.) 'Umar once said, "May Allah not purify the one who does not consider wiping over the turban to be purifying." Many hadith have been related on this topic by al-Bukhari, Muslim and others. Most of the scholars agree with them.

 Wiping over the front portion of the scalp and the turban:
Al-Mughirah ibn Shu'bah said that the Messenger of Allah, upon whom be peace, made ablution and wiped over the front portion of his scalp, his turban and his socks. (Related by Muslim.) There is, however, no strong hadith that he wiped over part of his head, even though al-Ma'idah: apparently implies it. It is also not sufficient just to wipe over locks of hair that proceed from the head or along the sides of the head.

Washing the feet and the heels:
This has been confirmed in mutawatir (continuous) reports from the Prophet, upon whom be peace, concerning his actions and statements. Ibn 'Umar said, "The Prophet lagged behind us in one of our travels. He caught up with us after we had delayed the afternoon prayer. We started to make ablution and were wiping over our feet, when the Prophet said, 'Woe to the heels, save them from the Hell-fire,' repeating it two or three times." (Related by al-Bukhari and Muslim.) Needless to say, the preceding obligations are the ones that Allah has mentioned in (al-Ma'idah 6).

Following the prescribed sequence:
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.