All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
It is not a condition to utter the intention with the tongue, because the intention is in the heart.
Ibn Qudamah said:
“The meaning of intention is to resolve to do something, i.e. to have the resolution in the heart to do something, and to be determined to do it without hesitation.” [End of quote]
The Fiqh Encyclopedia reads:
“The intention is: the firm resolution to do something [i.e. to have the resolution in the heart to do something, and to be determined to do it without hesitation] It is not a mere willingness or inclination to doing something. Rather, it has to be held with resolve and with no hesitation or conditioning that undermines such determination.”
Shaykh Ibn Taymiyyah said:
“The intention follows the knowledge. When a person knows what he is to do, then he must have intended it; it cannot be imagined, with the existence of knowledge of what he does, that he had no intention of doing it. The Imams (i.e. the Scholars) agreed that saying the intention loudly and repeating it is not permissible. Rather, the one who was used to do so, should be disciplined. He should be disciplined in a way that prevents him to worship (Allah) with innovations, and by harming the people by raising his voice…..” [End of quote]
Shaykh al-'Uthaymeen said in ash-Sharh al-Mumti':
“Issue: Some people say that the intention is difficult for him.”
The answer is that the intention is easy and leaving it is difficult.
If someone purifies himself (performs ablution) in his house and then walks to the mosque to perform the prayer, then there is no doubt that he has made the intention. What brought him to the mosque and made him to stand in the row and say the Takbeer (saying Allahu Akbar to start the prayer) is the intention to perform the prayer. Some scholars even said: If Allah had commissioned us to do an action without making the intention, it will be an unbearable task. If it was said: pray but do not make the intention of the prayer; perform ablution but do not make the intention to perform ablution, he could not do so. There is no action except with the intention. It is for this reason that Shaykh Al-Islam Ibn Taymiyyah () said: "The intention follows knowledge; if someone knows what he wants to do, then he has indeed intended it; it is not possible to do it without intention.” He has indeed said the truth . The evidence about this is the saying of the Prophet : “Indeed, actions are judged by intention.”; i.e. there is no action without intention.” [End of quote]
Therefore, it becomes clear to you, dear brother, that whoever intends to do something, then he has intended it. This does not take much effort, or to endeavor to have the intention. The intention does not need to be said in one’s mind or to be uttered by one’s tongue. Rather, it is the intention itself. Whoever intends to pray the ‘Asr prayer, for example, and he stands up and says the Takbeer while intending the ‘Asr prayer, then the intention is there, even if he does not say in his mind or with his tongue (I intend to pray 'Asr). Similarly, if one used water with the intention of performing ablution, or Ghusl, then the intention is there [i.e. he had intended to do so].
Allah knows best.