Concentration in Kerias Shema:
One is to be precise to recite every word of Kerias Shema This is a very great thing. Even if one is unable to pray the entire prayer without foreign thoughts, nevertheless he is to exert effort at the very least to accustom himself to say Shema without foreign thoughts. One who does so effects that the damaging forces separate from him and he is saved from bodily damage and damage to his soul.
 Keser Shem Tov 3, Tzavah of Baal Shem Tov