I didn't mean to imply that you made it up, Bitt. Sorry if it sounded like that. I agree that "zu" and "zur" are both grammatically correct.

Book of Common Prayer

That may explain it. I googled for "Asche zu Asche" and "Asche zur Asche" before my previous post to check that my claim wasn't subjective. "Asche zu Asche" won 12100:62. But for some strange reason the vast majority of links for "Asche zur Asche" were of a religious nature. The line "Erde zur Erde, Asche zur Asche, Staub zum Staube.", spoken before graveyards, was the most frequent to cross my path.
Now that you mentioned prayers and since there's another strange, dusty word in that line (Staube instead of the more common Staub) I'd say that it's old German which reminds me of thee and ye.... which in turn remind me of Pulp Fiction every time I hear them.