Stone Bones (Spell)

Transmutation
Level: Sorcerer 2, Wizard 2, Cleric 2,
Components: V, S, AF,
Casting Time: 1 action
Range: Touch
Target: Corporeal undead creature touched
Duration: 10 minutes/level
Saving Throw: Will negates (harmless)
Spell Resistance: Yes (harmless)
You cause the skeleton of the target corporeal undead to become thicker and as strong as stone.
This gives the target a +3 natural armor bonus to AC.
It has no effect if the creature has no skeleton

Source: Spell Compendium p. 209