Ring of Regeneration

Ring, major, very rare (requires attunement)

While wearing this ring, you regain 1d6 hit points every 10 minutes, provided that you have at least 1 hit point. If you lose a body part, the ring causes the missing part to regrow and return to full functionality after 1d6 + 1 days if you have at least 1 hit point the whole time.

Source: Dungeon Master’s Guide p. 191. Available in the SRD