...and he hath said that no unclean thing can inherit the kingdom of heaven.This is a recurrent theme in the Book of Mormon. For instance, Nephi is saying the same thing to his brothers in 1st Nephi 15:34 and Jesus is saying it to his disciples in 3rd Nephi 27:19. But at this point in the book (Alma 11), there has not been a clear example of God saying this. Also, this phrase is not found anywhere in the Old Testament. So when Amulek is saying "he [God] hath said", what is he referring to? Fortunately, we have search functions which enable us to find answers to these questions much more easily than before.
In the Pearl of Great Price, Moses 6:55, we read that "the Lord spake to Adam" and in 6:57, he says that
all men, everywhere, must repent, or they can in nowise inherit the kingdom of God, for no unclean thing can dwell there.So it seems that the words which the Lord spoke to Adam according to the pearl of Great Price, which have been lost from our current version of the Old Testament, were found on the brass plates.