Direct counterargument: I would phrase my attitude to ethics as: "I have decided that I want X to happen as much as possible, and Y to happen as little as possible." I'm not "believing" anything - just stating goals. So there's no faith required.

Reflective counterargument: But even if God did say so*, why should we obey Him? There are a number of answers, some based on prior moral concepts (gratitude for Creation, fear of Hell, etc.) and some on a new one (variations on "God is God and therefore has moral authority") but they all just push the issue of your ultimate basis for morality back a step. They don't solve the problem, or even simplify it.

*Incidentally, what does it mean for an all-powerful being to say something? The Abrahamic God is the cause for literally everything, so aren't all instructions written or spoken anywhere by anyone equally "the speech of God"?