As explained under 1 Nephi 11:31, in earlier English the preposition of was used as the agentive preposition instead of the modern by. Here in Mormon 8:1, we have an instance where the original text reads of but the typesetter for the 1841 British edition, unintentionally it would appear, replaced the of with by. The LDS text has maintained the modern use of by, but the critical text will restore the of.
Elsewhere in the text, given the passive form of the verb command, there are ten occurrences with the agentive preposition of and three with the by, so the original of in Mormon 8:1 is perfectly possible. Nonetheless, it should be pointed out that all ten of the occurrences with of involve a command from the Lord, while all three with by involve a command from a person, not the Lord. One of these three specifically refers to the command of an earthly father:
Nonetheless, here in Mormon 8:1 of is clearly possible and quite understandable. For an example where an original agentive by was replaced by the archaic of, see under 1 Nephi 22:5.
Summary: Restore the original agentive preposition of in Mormon 8:1 (“I have been commanded of my father”); this archaic usage with of occurs quite frequently in the Book of Mormon text.