In Salah it is necessary to move the lips and tongue when reciting the different types of supplications, eg: Takbir Tahrimah (The Takbir to commence Salah), Qiraat, Tashahud, Salam, etc. If one does not verbally recite the necessary words by moving the lips and tongue but merely recites it in his heart, the Salah will be invalid.
It is necessary for men to cover the area between the naval and knees at all times. Exposing this area is sinful and if it is exposed during Salaah, the Salaah is invalid. Many modern fashion trends cause parts of this area, more specifically parts of the hip and below, to be exposed when one goes into Ruku or Sajda. Brothers are requested to take heed and wear modest clothing.