Do as your mouth spoke

“Lo Yachel Devaroi, K’Chol HaYotzai MiPicha Yaaseh”, do not profane your words; do as your mouth spoke. The Torah tells us that we must keep our word and not violate it.

Rav Levi Yitzchok MiBerditchev in the Kedushas Levi makes a play on the words to explain how mortal people can perform miracles. He reads the words as follows. If “Lo Yachel Devaroi”, you do not profane your words, then they will be holy and meaningful. Therefore, “K’Chol HaYotzai MiPicha Yaaseh” whatever comes out of your mouth will happen.

This is the concept of “Tzadik Gozer, VHaKadosh Boruch Hu Mikayem”, a tzaddik decrees and Hashem makes it happen . He further explains that this why the Parsha is called Matos. Matos also means to turn (Netia). When a person watches his mouth, Hashem turns the Midas HaDin into Midas HaRachamim.
http://shortvort.com/matos-parasha/10865-a-chassidishe-vort