CHĀNDOGYA UPANISHAD · 8.3.3

स वा एष आत्मा हृदि ।
तस्यैतद् एव निरुक्तं हृद्य् अयम् इति तस्माद् धृद् अयम् ।
अहर् अहर् वा एवंवित् स्वर्गं लोकम् एति ॥

sa vā eṣa ātmā hṛdi ।
tasyaitad eva niruktaṃ hṛdy ayam iti tasmād dhṛd ayam ।
ahar ahar vā evaṃvit svargaṃ lokam eti ॥

'That Self abides in the heart. And this is the etymological explanation. The heart is called hrid-ayam, instead of hridy-ayam, i. e. He who is in the heart. He who knows this, that He is in the heart, goes day by day (when in sushupti, deep sleep) into heaven (svarga), i.e. into the: Brahman of the heart.

F. Max Müller · The Upanishads, Sacred Books of the East 1 (1879)