Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.027A.

This is the desire to do the action and to please Allah by following His command. It is purely an act of the heart, for the tongue (verbal pronouncement, and so on) has nothing to do with it. To pronounce it is not part of the Islamic law. That the intention is obligatory is shown in the following: ‘Umar related that the Prophet, upon whom be peace, said, “Every action is based on the intention (behind it), and everyone shall have what he intended…” (Related by “the group.”)