How is the intention made in the heart, and is it valid to pronounce it without moving the tongue?

Intention (niyyah) is the resolve to perform an action. It suffices for the worshipper to intend in his heart to perform the prayer he desires, such as intending the commencement of the Dhuhr or Asr prayer. Verbalizing the intention is permissible according to some scholars, but the preponderant opinion is that it is not prescribed.