Rhetorical: Alma reinforces the temporal aspect by another appear to the ultimate “father.” Alma reminds them of the declaration of Lehi that his descendents would prosper in the land if they kept the commandments. Mormon is citing 2 Nephi 1:20. Obviously there was no referential system on the plates so that he could tell the people of Ammonihah that he was making the citation. Alma clearly expects them to know the reference from the simple recitation and attribution to Lehi.
Textual: The reference is to Lehi’s blessing of his sons:
2 Ne. 1:19-20
19 O my sons, that these things might not come upon you, but that ye might be a choice and a favored people of the Lord. But behold, his will be done; for his ways are righteousness forever.
20 And he hath said that: Inasmuch as ye shall keep my commandments ye shall prosper in the land; but inasmuch as ye will not keep my commandments ye shall be cut off from my presence.