From Dovid Sears, “The Path of the Baal Shem Tov” (Jason Aronson), p. 119.
The Baal Shem Tov on Silence
“The fence for wisdom is silence” (Avos 3:13). When one is silent, he is able to bind himself to the World of Thought, which is called “wisdom” (Kesser Shem Tov 225).
“Whoever increases words brings about sin (chet)” (Avos 1: 17). [The Hebrew word for sin (chet) means a lack or deficiency (see Rashi on Bereishis 31:39). [Thus, the Mishnah implies that] even when one discusses the wisdom of Torah with others, silence would still be better. With silence, one can contemplate the greatness of God and bind oneself to Him more completely than through speech. At times one may lie in bed and appear to be sleeping, when one s really meditating upon the Creator, may He be blessed (Tzava’as ha-Rivash 133).
If a person is subjected to ridicule because of his manner of prayer or such things, he should not respond, even with pleasant words. In this way, he may avoid both strife and the feeling of self-importance which comes from forgetting about the Creator, may He be blessed. Thus, our Sages taught that a man's silence is greater than his speech, for it brings him to humility (Megillah 18a) (Tzava’as ha-Rivash 49).
One must often serve God with his soul alone—that is, in thought—and the body must remain still, so that it does not become weak from exertion (Tzava’as ha-Rivash 104).