Arcane Toughness

 
Prerequisite(s): Arcane Caster level 3+, Toughness
 
Benefit: You can expend a prepared spell or spell slot as an immediate action when you are reduced to 0 or fewer hit points. You heal a number of points equal to the level of the spell or spell slot that was sacrificed. You cannot use this feat to negate effects that disable you but do not cause hit point damage. Even if this healing does not bring you above negative hit points, you are still stabilized.
 
Source: Players Handbook II