Chandogya Upanishad 8.33

अथ यत्रैतदस्माच्छरीरादुत्क्रामत्यथैतैरेव रश्मिभिरूर्ध्वमाक्रमते स ओमिति वा होद्वा मीयते स यावत्क्षिप्येन्मनस्तावदादित्यं गच्छत्येतद्वै खलु लोकद्वारं विदुषां प्रपदनं निरोधोऽविदुषाम् ॥ ५ ॥

8.6.5 — atha yatraitadasmachchhariradutkramatyathaitaireva rashmibhirurdhvamakramate sa omiti va hodva miyate sa yavatkshipyenmanastavadadityam gachchhatyetadvai khalu lokadvaram vidusham prapadanam nirodho'vidusham || 5 ||

But when he departs from this body, then he departs upwards by those very rays (towards the worlds which he has gained by merit, not by knowledge); or he goes out while meditating on Om (and thus securing an entrance into the ​loka). And while his mind is failing, he is going to the sun. For the sun is the door of the world (of Brahman). Those who know, walk in; those who do not know, are shut out. There is this verse: 'There are a hundred and one arteries of the heart; one of them penetrates the crown of the head; moving upwards by it a man reaches the immortal; the others serve for departing in different directions, yea, in different directions.'

'परन्तु जब वह इस शरीर से उत्क्रान्त होता है, तब वह इन्हीं रश्मियों से ऊपर की ओर जाता है। 'ओम्' कहते हुए अथवा कहे बिना वह निकलता है; जितने क्षण में मन को प्रेरित किया जा सके, उतने में वह आदित्य तक पहुँचता है। यही विद्वानों के लिये लोक-द्वार (प्रवेश-मार्ग) है और अविद्वानों के लिये निरोध (अवरोध)।'