सर्वे वेदा यत्पदमामनन्ति तपाँ सि सर्वाणि च यद्वदन्ति ।
यदिच्छन्तो ब्रह्मचर्यं चरन्ति तत्ते पद्ँ संग्रहेण ब्रवीम्योमित्येतत् ॥ १५ ॥

sarve vedā yatpadamāmananti tapām̐si sarvāṇi ca yadvadanti |
yadicchanto brahmacaryaṃ caranti tatte padm̐ saṃgraheṇa bravīmyomityetat || 15 ||

15. The goal which all the Vedas uniformly extol, which all acts of tapas speak of, and wishing for which men lead the life of a Brahmacharin, that goal I tell you briefly—It is this—Om.

Shankara’s Commentary:

To him, who had thus questioned, Death explained the thing asked for and also something else, i.e., the worship of ‘Om.’ What praiseworthy goal all Vedas without break, i.e., with one voice, declare, to which goal all acts of iapas are intended to lead, and desirous of which men live in the residence of their preceptor, or practise other kinds of Brahmacharya to attain the Brahman, that goal which you wish to learn, I shall tell you briefly. It is this Om’. The goal which you wish to learn is the goal which is denoted by the word ‘Om,’ and of which the word ‘Om’ is a substitute (Pratika).