Ring of Regeneration
Ring, very rare, requires attunement · 2024 rules, SRD 5.2
While wearing this ring, you regain 1d6 Hit Points every 10 minutes if 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.
Also in the 2014 rules (SRD 5.1).