Assalaamu Alaikum Wa Rahmatulaah
He (sallallaahu 'alaihi wa sallam) used to say: All actions are by intention, and every man shall have what he intended.
Bukhaari, Muslim and others. It is given in Irwaa' (no. 22)
The worshipper must have the intention of praying the prayer for which he is standing. He must have the intention in his heart of performing a specific prayer, such as the fard (obligatory prayer) of Zuhr or of ‘Asr, or the Sunnah of those prayers. This is a condition or pillar (essential part) of the prayer, but uttering the intention verbally is a bid’ah which goes against the Sunnah, which was not suggested by any of the imams who are followed.
You can read a really good book on prayer online: