Surprisingly, this rule is fairly clear: When a noun ends in an “s” the possessive is formed by placing the apostrophe outside the word: Moses’. The confusion here probably comes from the pronunciation of the final syllable in Moses, “-es”, which is pronounced the same as many possessives: hous: house’s, face: face’s, etc. Under no condition is “Moses’s” a correct possessive form. Often the confusion can be avoided by a slight change of wording: (“the words of Moses”). Remembering that the possessive apostrophe is a written distinction, not a voiced one, may help. The apostrophe as a signal of a contraction (didn’t, couldn’t, etc.) should not be confused with the apostrophe for signaling possession.
You will find that some nouns, especially proper nouns, especially when there are other -s and -z sounds involved, turn into clumsy beasts when you add another s ... you're better off with "Mrs. Chambers' estate." (Guide to Grammar and Writing, Capital Community College)