Brihadaranyaka-Upanishad: Fifth Adhyâya, Eleventh Brahmana

1. This is indeed the highest penance, if a man, laid up with sickness, suffers pain. He who knows this, conquers the highest world.

This is indeed the highest penance, if they carry a dead person into the forest. He who knows this, conquers the highest world.

This is indeed the highest penance, if they place a dead person on the fire. He who knows this, conquers the highest world.