cast in stone
Also, etched in stone. Definite, fixed, as in We may choose to stay longer-our plans aren't cast in stone, or When Carl sets an agenda you can safely assume it's etched in stone. Both expressions allude to sculpture, with the first, from the early 1500s, using the verb cast in the sense of pouring and hardening some material into a final form, and the second cutting or corroding a permanent design.