BṚHADĀRAṆYAKA UPANISHAD · 1.5.4

त्रयो लोका एत एव ।
वाग् एवायं लोको मनो ऽन्तरिक्षलोकः प्राणो ऽसौ लोकः ॥

trayo lokā eta eva ।
vāg evāyaṃ loko mano ’ntarikṣalokaḥ prāṇo ’sau lokaḥ ॥

These are the three worlds: earth is speech, sky mind, heaven breath.

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