"Go to Bethel, and sin; To Gilgal, and sin more. Bring your sacrifices every morning, Your tithes every three days,
Despite postulations such as those, it is quite clear that the Law of Moses was indeed in existence when Amos wrote, and it was known wherever Jews lived; and this prohibition in that law would never have been mentioned in the context here unless that is the way it was.  As for Smith's notion that "leaven" was the sign of anyone's affluence (!), one would be hard-pressed indeed to come up with anything more ridiculous.  The children of Israel, while in slavery, had plenty of leaven, hence, the prohibition that the Passover should be celebrated with "unleavened" bread.