Likutei Moharan 75:5/ליקוטי מוהר"ן ע״ה:ה׳

15. And a person must speak words of Torah or prayer until the body becomes negated and nullified. This is the explanation of the verse “and they become one flesh”—the body should become one with the speech to the point where it becomes negated. How does he achieve this? Through fear. For our Sages, of blessed memory, taught: The body is powerful, [yet] dread breaks it. And “dread” is the aspect of “the Dread of Yitzchak”—i.e., fear.