Bonded Familiar( Player's Handbook II, p. 75)
You enjoy a stronger than normal magical bond with your familiar, granting you access to two special abilities.
As long as your familiar is within 30 feet of you, the two of you can share the damage from a single deadly attack. Once per day, if an attack would drop your familiar to 0 or fewer hit points, you can instead choose to accept that damage. In addition, once per day, your familiar can choose to take the damage from a single attack or effect that would reduce you to 0 or fewer hit points. This ability applies only to attacks or effects that deal hit point damage. You or your familiar could not absorb the effect of a spell such as flesh to stone.