A Tzaddik Decrees

"He shall not desecrate his word; according to whatever comes from his mouth shall he do" [Matos 30:3]

Rashi explains: "Lo yacheil devaro" - [This has the same meaning] as 'he shall not profane [yechalel] his word' [that is] he shall not make his words mundane."

On this verse, R' Menachem Mendel of Kosov remarked: When an individual is careful to speak only of holy matters and consistently avoids speaking of mundane matters, then Hashem rewards him by fulfilling the man's blessings or prayers.  Thus, Chazal asserts "A tzaddik decrees and HaKadosh Baruch Hu fulfills".

This idea, concluded the Rebbe, is hinted at in the verse "Lo yacheil devaro" - A person who does not make his words profane merits reaching the level where "according to whatever comes from his mouth He will do" - A tzaddik decrees and HaKadosh Baruch Hu fulfills."

Source: Rabbi Yisrael Bronstein