Cat litter (clumping or clay) is not safe for rabbits. The only type of "cat litter" that's safe is Yesterday's News (compressed paper pellets) or things like Feline Pine (compressed pine pellets).
We are as gods to the beasts of the fields. We order the time o' their birth and the time o' their death. Between times, we ha' a duty. - Terry Pratchett.
"Men have forgotten this truth", said the fox, "But you must not forget it. You become responsible, forever, for what you have tamed. - Antoine de Saint-Exupéry