Praise be to Allah.
The intention (niyyah) is essential for all acts of worship; no act of worship – including wudoo’ – is valid without the intention.
Al-Nawawi (may Allaah have mercy on him) said:
The intention is an essential condition of wudoo’, ghusl and tayammum being valid, and there is no difference of opinion among us concerning that. This is the view of Maalik, al-Layth ibn Sa’d, Ahmad ibn Hanbal and Dawood. They quoted as evidence the words of Allaah (interpretation of the meaning):
“And they were commanded not, but that they should worship Allaah, and worship none but Him Alone (abstaining from ascribing partners to Him)”
Worshipping none but Allaah with sincere devotion is an action of the heart; this is intention, and the command here shows that it is obligatory.
From the Sunnah, we have the words of the Prophet (peace and blessings of Allaah be upon him): “Actions are but by intentions.”
What is meant is that the ruling on an action is based on the intention behind it.
Another piece of evidence is the hadeeth in which the Prophet (peace and blessings of Allaah be upon him) said: “Every person will have but that which he intended.” If he did not intend to do wudoo’ then he has not done wudoo’, and so on.
End quote from al-Majmoo’, 1/356. See also al-Mughni, 1/156
It should be noted that the seat of the intention is the heart, and it is not prescribed to utter it out loud.
See question no. 13337.
It is better to form the intention when starting to do wudoo’ or shortly before, so that the intention will cover all parts of wudoo’. What is obligatory is to have the intention when starting to do the obligatory parts of wudoo’.
Ibn Qudaamah said in al-Mughni, 1/159:
The intention must precede the entire act of purification, because it is an essential condition thereof, so it is regarded as being essential throughout. If any of the obligatory parts of the act of purification take place before the intention, they do not count. It is mustahabb to form the intention before washing one's hands, so that the intention will cover both the Sunnah parts of the purification and the obligatory parts. If a person washes his hands before forming the intention, he will be like one who has not washed his hand. It is permissible to form the intention shortly before purifying oneself… but if there is a long time in between, that is not sufficient. End quote.
Shaykh Ibn ‘Uthaymeen said in al-Sharh al-Mumti’ (1/140):
The intention is either of two things:
1 – Either it is Sunnah, which is if it is done before a Sunnah act of purification that comes before a waajib act’
2 – Or it is required when doing the first obligatory action. End quote.
Based on this, it is better to form the intention before starting to do wudoo’, and it is obligatory to have the intention when doing the first of the obligatory actions of wudoo’. The scholars differed as to which is the first obligatory action of wudoo’. It was said that it is saying Bismillaah, or rinsing out the mouth – which is the correct view – or washing the face. See questions no. 21241 and 11497.
But if a person forms the intention with the first obligatory action of wudoo’, he will not be rewarded for whatever Sunnahs of wudoo’ he did before that, such as saying Bismillaah and washing the hands three times, as stated above in the words of Ibn Qudaamah (may Allaah have mercy on him).
Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allaah have mercy on him) said:
It is proven from the Prophet (peace and blessings of Allaah be upon him) that the first part of wudoo’ is to wash the hands three times, with the intention of doing wudoo’, and to say Bismillaah.
Majmoo’ Fataawa al-Shaykh Ibn Baaz, 10/98
And Allaah knows best.