Do You Need to Utter Intention for Acts of Worship?
Question

Should a Muslim utter the intention (Niyyah) when he starts to do an act of worship, such as saying, “I intend to do Wudu’”, “I intend to pray”, “I intend to fast” and so on?
Praise be to Allah.Shaykh Al-Islam Ibn Taymiyah (may Allah have mercy on him) was asked about the intention when starting to do an act of worship such as praying etc., do we need to utter it verbally, such as saying, “I intend to pray, I intend to fast”? 
He (may Allah have mercy on him) replied: 
“The intention of purifying oneself by doing Wudu, Ghusl or Tayammum, of praying, fasting , paying Zakah, offering expiation and other acts of worship does not need to be uttered verbally, according to the consensus of the Imams of Islam. Rather the place of intention is the heart, according to the consensus among them. If a person utters something by mistake that goes against what is in his heart, then what counts is what he intended, not what he said. 
No one has mentioned any difference of opinion concerning this matter, except that some of the later followers of Ash-Shafi`i expressed approval of that, but some of the leaders of this Madhhab said that this was wrong. But in the dispute among the scholars as to whether it is recommended to utter one’s intention, there are two points of view. Some of the companions of Abu Hanifah, Ash-Shafi`i and Ahmad said that it is recommended to utter the intention so as to make it stronger. 
Some of the companions of Malik, Ahmad and others said that it is not recommended to utter it, because that is an innovation. It was not narrated that the Messenger of Allah (peace and blessings of Allaah be upon him) or his Companions did it or that he commanded anyone among his Ummah to utter the intention. That is not known from any of the Muslims. If that had been prescribed then the Prophet (peace and blessings of Allah be upon him) and his Companions would not have neglected it, as it has to do with worship which the Ummah does every day and night. 
This is the more correct view. Indeed, uttering the intention is an irrational thinking and falling short in religious commitment. In terms of falling short in religious commitment, that is because it is an innovation. 
In terms of irrational thinking, that is because it is like a person who wants to eat some food saying, “I intend to put my hand in this vessel, take out a morsel of food, put it in my mouth and chew it, then swallow it, and eat until I have had my fill.” This is sheer foolishness and ignorance. 
Intention is connected to knowledge. If a person knows what he is doing, then he has obviously made an intention. It cannot be imagined, if he knows what he wants to do, that he has not formed an intention.
The Imams agreed that speaking the intention out loud and repeating it is not prescribed in Islam, rather the person who has made this a habit should be disciplined and told not to worship Allah by following Bid`ah and not to disturb others by raising his voice. 
For more details, please see the following answers: 309385 , 20742 , 39689 , 223721 , and 20193 .
And Allah knows best.