The implication is that he had already prepared himself through fasting, since he could immediately perform the healing when called upon but did not know ahead of time that his disciples would fail.
Book of Mormon Context: The Book of Mormon context of fasting includes aspects of both mourning (Alma 28:6, 30:2) and spiritual preparation in conjunction with prayer (Alma 6:6, 17:3). The text contains no information about particular attire or other visible manifestations to accompany Nephite fasting. Thus, the applicability of this particular element in the sermon is unknown.
Comparison: There are no changes from the Matthean text.