The Laws of Prayer (The morning services) (31)

1. During the Amidah one must concentrate on the translation of the words which he says, and think in his heart that if he would be speaking in front of a king of flesh and blood he would surely organize his words well before speaking and concentrate properly on them so that he will not stumble or make an error, and how much more so when standing before the King of kings who knows all of our thoughts. (Shulchan Aruch and Mishne Berurah 98)

2. If another thought enters his mind in the middle of the prayer, he should stop praying until that thought has left his mind. (ibid)