"Go to Bethel, and sin; To Gilgal, and sin more. Bring your sacrifices every morning, Your tithes every three days,
Now, in the light of these very specific, yet incidental, references to the Law of Moses, it is impossible logically to support the notion that no such law existed when Amos wrote.  If no such law had been in existence, it would have been necessary for him to explain why the things mentioned were sinful; and the fact of no explanation being offered proves the prior existence of the Mosaic law which included those prohibitions.  The mere denials of scholars who wish to think otherwise are worthless in such a clear-cut demonstration as is found here.