Chāndogya Upaniṣad 2.23

2.23.1

trayo dharmaskandhāḥ \
yajño 'dhyayanaṃ dānam iti prathamaḥ \
tapa eva dvitīyaḥ brahmacāry: ācāryakulavāsī tṛtīyo (S S ))\ sarva ete puṇyalokā bhavanti \
brahmasaṃstho 'mṛtatvam eti \\223.1\

1. There are three branches of duty. Sacrifice, study of the Vedas, alms-giving—that is the first. (2) Austerity, indeed, is the second. A student of sacred knowledge (brahmacārin) dwelling in the house of a teacher, settling himself permanently in the house of a teacher, is the third.

All these become possessors of meritorious worlds. He who stands firm in Brahma attains immortality.

2.23.2

prajāpatir lokān abhyatapat \
tebhyo 'bhitaptebhyas trayī vidyā saṃprāsravat \
tām abhyatapat \
tasyā abhitaptāyā etāny akṣarāṇi saṃprasrvanta bhūr bhuvaḥ svar iti \\223.2\

2 (3). Prajāpati brooded upon the worlds. From them, when they had been brooded upon, issued forth the threefold knowledge. He brooded upon this. From it, when it had been brooded upon, issued forth these syllables: bhūr, bhuvaḥ, svar.

2.23.3

tāny abhyatapat \
tebhyo 'bhitaptebhya oṃkāraḥ saṃprāsravat \
tad yathā śaṅkunā sarvāṇi parṇāni saṃtṛṇṇāny evam oṃkāreṇa sarvā vāk saṃtṛṇṇā \
oṃkāra evedaṃ sarvam oṃkāra eva idaṃ sarvam \\223.3\

3 (4). He brooded upon them. From them, when they had been brooded upon, issued forth the syllable Om. As all leaves are held together by a spike, so all speech is held together by Om. Verily, Om is the world-all. Verily, Om is this world-all.