In Alma 50:20-21, Mormon repeats a covenant promise spoken to father Lehi by the Lord, and notes that it has been verified to the Nephites:
Blessed art thou and thy children; and they shall be blessed, inasmuch as they shall keep my commandments they shall prosper in the land. But remember, inasmuch as they will not keep my commandments they shall be cut off from the presence of the Lord. And we see that these promises have been verified to the people of Nephi . . .
The reader should note that this covenant promise is repeated throughout both the Small Plates and Mormon's abridgment (1 Nephi 4:14; 2 Nephi 1:20, 4:4; Omni 1:6; Mosiah 1:7, 2:22, 2:31; Alma 9:13, 36:1, 37:13, 38:1, 48:25, 50:20).
The reader should also note that a similar covenant promise was given to Moses:
Wherefore ye shall do my statutes, and keep my judgments, and do them; and ye shall dwell in the land in safety. And the land shall yield her fruit, and ye shall eat your fill, and dwell therein in safety. (Leviticus 25:18-19)
[Alan C. Miner, Personal Notes]