'Umar Ibn Al-Khaṭṭāb (may Allah be pleased with him) narrated that he heard Allah's Messenger (may the peace and blessings of Allah be upon him) saying:
“Actions are only [considered] according to [their] intentions, and everyone will have [only the reward of] what they intended..." [Bukhari: 1; Muslim: 1907]
The worshipper must have the intention in his heart of performing the prayer for which he is standing. The intention is esentially an awareness of the action in the heart, which precedes the action itself, such that if a person were to be asked: "Why are you standing here?" He would be able to respond, "I am going to pray such-and-such a prayer." If the person would be unable to answer the question, this is a sign that they have not made the proper intention. The intention is a condition or pillar of the prayer.
There are no authentic narrations from the Prophet (may the peace and blessings of Allāh be upon him) which indicate that the intention for the prayer should be uttered verbally, or that there are certain words that are said silently in the heart. To say the intention out loud is an innovation in the religion which goes against the authentic guidance of the Prophet (may the peace and blessings of Allah be upon him).