ŚVETĀŚVATARA UPANISHAD · 1.14

स्वदेहम् अरणिं कृत्वा प्रणवं चोत्तरारणिं ।
ध्याननिर्मथनाभ्यासाद् देवं पश्येन् निगूढवत् ॥

svadeham araṇiṃ kṛtvā praṇavaṃ cottarāraṇiṃ ।
dhyānanirmathanābhyāsād devaṃ paśyen nigūḍhavat ॥

By making his body the under-wood, and the syllable Om the upper-wood, man, after repeating the drill of meditation, will perceive the bright god, like the spark hidden in the wood.

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