Tinyana 51 · תנינא נ״א

The evil inclination continually comes against a person and arouses him to what it arouses him. And even if a person does not listen to it, but turns his back on [the evil inclination], it nevertheless comes against him a second, third and fourth time, and more. But if a person is strong-willed and determined in opposing the evil inclination, and does not turn to it in the least, then the evil inclination leaves him and goes on its way.

The same is true of prayer; in regard to thoughts that come to distract, it is exactly the same. The thought comes a number of times, time after time, in order to confuse. One has to be strong, so as not to pay attention to it under any circumstances. And in this way, it leaves. See elsewhere regarding this.