One basic halakhic rule for public reading of the Torah is that no fewer than three verses at a time be read. As a corollary to this, there is a specific rule regarding parashot: One may not leave off reading less than three verses before the end of a parashah, nor may one end off reading by starting a new parashah but leaving off less than three verses from its beginning.The talmudic source for this is Megillah 22a. In later halakhic literature, these rules are discussed in Orah Hayyim 138.
