Brihadaranyaka Upanishad 5.11.1

5.11.1

etadvai paramaṃ tapo yadvyāhitastapyate paramam̐ haiva lokaṃ jayati ya evaṃ vedaitadvaiparamaṃ tapo yaṃ pretamaraṇyam̐ haranti paramam̐ haiva lokaṃ jayati ya evaṃ vedaitadvaiparamaṃ tapo yaṃ pretamagnāvabhyādadhati paramam̐ haiva lokaṃ jayati ya evaṃ veda

This indeed is excellent austerity that a man suffers when he is ill. He who knows as above wins an excellent world. This indeed is excellent austerity that a man after death is carried to the forest. He who knows as above wins an excellent world. This indeed is excellent austerity that a man after death is placed in the fire. He who knows as above wins an excellent world.

5.11.1.p1

iti pañcamādhyāya ekādaśaṃ brāhmaṇam