BṚHADĀRAṆYAKA UPANISHAD · 5.11.1

एतद् वै परमं तपो यद् व्याहितस् तप्यते ।
परमं हैव लोकं जयति य एवं वेद ।
एतद् वै परमं तपो यं प्रेतम् अरण्यं हरन्ति ।
परमं हैव लोकं जयति य एवं वेद ।
एतद् वै परमं तपो यं प्रेतम् अग्नाव् अभ्यादधति ।
परमं हैव लोकं जयति य एवं वेद ॥

etad vai paramaṃ tapo yad vyāhitas tapyate ।
paramaṃ haiva lokaṃ jayati ya evaṃ veda ।
etad vai paramaṃ tapo yaṃ pretam araṇyaṃ haranti ।
paramaṃ haiva lokaṃ jayati ya evaṃ veda ।
etad vai paramaṃ tapo yaṃ pretam agnāv abhyādadhati ।
paramaṃ haiva lokaṃ jayati ya evaṃ veda ॥

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.

F. Max Müller · The Upanishads, Sacred Books of the East 15 (1884)