What is the ruling on a wife telling her husband – when he thanks her and prays for her health – "Go and bathe with it," and her saying, "God forbid that I would mean God with my words; rather, I mean you, as I am not in need of your expression and prayer"? Are there any consequences for this? And is it considered contempt and mockery of God's will?

If the wife does not intend any false meanings with this statement, then there is no mockery of Allah Almighty's will in it. However, she should choose good words, return greetings with something better, and avoid inappropriate expressions, especially with her husband; in obedience to Allah Almighty's saying: "And tell My servants to say that which is best. Indeed, Satan incites [discord] among them. Indeed Satan is ever, to man, a clear enemy."