Body Training (5e Feat)
From D&D Wiki
Prerequisites: Must have a hit die of 1d8 or lower
You have trained your body to an extreme extent, hardening your body. Choose one class in which you have levels, that has a hitdie of 1d8 or lower. All of your hitdice from that class are increased by one step, although your hitpoints do not change. When gaining hitdie for that class, roll your extra hitpoints normally, and increase the hitdie afterwards.