Wodhu - Ablution
The procedure of Wodhu is explicitly explained in Quran.
"Wash your faces and your hands..., rub your heads and your feet..."
"O ye who believe! when ye prepare for prayer, wash your faces, and your
hands (and arms) to the elbows; Rub(masa) your heads (with water); and (rub-masa) your
feet to the ankles.
Allah swt has explicitely instructed to wash certain parts and rub(masa) certain parts
of the face and hands during Wodhu. He has not instructed all the parts to be washed only.
If Wodhu is not done according to His instructions then prayers will be effected and could
be void totally.
Shia Muslims wash their face, then their hands till elbows and do masa of head and
feet until ankles exactly as Allah instructed.
Sunni Muslims wash their face, then their hands till elbows and then wash their head and
feet until ankles which contradicts with Holy Quran.
Quran, using WAW of ATF, clearly says that you should rub (masa) your Feet. Sunni
Muslims wash their feet during Wodhu. But Allah says (5:6): "Wash your faces
and your hands..., rub (masa) your heads and your feet..."
Allah, exalted did not repeat the verb for "feet", and joined "heads" and
"feet" together under one verb "Rub", using Waw of Atf. This is exactly
what he did for "faces" and "hands" that came under one verb "Wash".
Muslims believe that waw al-'aTf requires that al-ma'Toof and al-ma'Toof
'alaih share the same status in i'rab ie if the ma'Toof ( in this case heads)
has a kasra, the ma'Toof 'alaih should also have a kasra. In this case, the
ma'Toof has a kasra, but the word feet has a fatHa indicating that it is
ma'Toofa 'ala the word wujuhakum and aydiyakum.
But in fact the reason that 'S' in 'ruusikum' has Kasrah is because
of the 'B' at the beginning of the 'beruusikum', and nothing else.
Moreover, when Allah uses a new verb, the old verb is already knocked out,
and you can not come back to it, because this is A NEW SENTENCE. There is
no possible way to use the verb of a previous sentence, when that sentence
has been already finished and a new sentence is started with a new verb.