Chandogya Upanishad 8.19

स वा एष आत्मा हृदि तस्यैतदेव निरुक्तꣳ हृद्ययमिति तस्माद्धृदयमहरहर्वा एवंवित्स्वर्गं लोकमेति ॥ ३ ॥

8.3.3 — sa va esha aatma hridi tasyaitadeva niruktam hridyayamiti tasmaddhridayamaharaharva evamvitsvargam lokameti || 3 ||

'That Self abides in the heart. And this is the etymological explanation. The heart is called hridayam, instead of hrzdy-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.

'यह आत्मा हृदय में है। इसका निरुक्त यही है — 'हृदि-अयम्' इसलिये 'हृदयम्' — 'जो हृदय में है, वह।' जो ऐसा जानता है, वह दिन-प्रतिदिन (सुषुप्ति में) स्वर्ग-लोक को प्राप्त करता है।'