At this point the Savior reprises and expands upon the saying that closed the formal Sermon on the Mount section (3 Nephi 14:24-27). The way that saying is transformed here is most easily seen when the two are directly compared:
3 Nephi 14:24
24 Therefore, whoso heareth these sayings of mine and doeth them, I will liken him unto a wise man, who built his house upon a rock—
3 Nephi 14:25
25 And the rain descended, and the floods came, and the winds blew, and beat upon that house; and it fell not, for it was founded upon a rock.
3 Nephi 18:12
12 And I give unto you a commandment that ye shall do these things. And if ye shall always do these things blessed are ye, for ye are built upon my rock.
Both sayings have two parts. The first part deals with those who do the commandments. The second likens those who do or keep the commandments to being built upon a rock.
What follows is therefore a comparison of the types of foundations upon which one might build. Absent from this most recent one is the explicit explanation of the correctness of building upon a rock. Of course that is implied, but the formal structure of the first saying is only referenced here, not replicated.
3 Nephi 14:26-27
26 And every one that heareth these sayings of mine and doeth them not shall be likened unto a foolish man, who built his house upon the sand—
27 And the rain descended, and the floods came, and the winds blew, and beat upon that house; and it fell, and great was the fall of it.
3 Nephi 18:13
13 But whoso among you shall do more or less than these are not built upon my rock, but are built upon a sandy foundation; and when the rain descends, and the floods come, and the winds blow, and beat upon them, they shall fall, and the gates of hell are ready open to receive them.
In this most recent restating we do not have the obvious contrast between those who do and those who do not. Nevertheless, it is clear that those who build upon a sandy foundation are the antithesis of those who build on rock. Apparently, since this occasion serves as a refresher for the first, the full structural relationship is not required. Verse 14 simply repeats the premise that began the reference.