Full Idea
That which by its own nature is something distinct from supreme good, cannot be supreme good. ..It is impossible for anything to be by nature better than that from which it is derived, so that which is the origin of all things is supreme good.
Gist of Idea
God is the supreme good, so no source of goodness could take precedence over God
Source
Boethius (The Consolations of Philosophy [c.520], III.X)
Book Reference
Boethius: 'The Consolations of Philosophy', ed/tr. Watts,V.E. [Penguin 1969], p.101
A Reaction
This is the contortion early Christians got into once they decided God had to be 'supreme' in the moral world (and every other world). Boethius allows a possible external source of all morality, but then has to say that this source is morally inferior.