KAṬHA UPANISHAD · 1.2.15

सर्वे वेदा यत्पदमामनन्ति
तपाꣳसि सर्वाणि च यद्वदन्ति ।
यदिच्छन्तो ब्रह्मचर्यं चरन्ति
तत्ते पदꣳ सङ्ग्रहेण ब्रवीम्योमित्येतत् ॥ १५॥

sarve vedā yatpadamāmananti
tapāꣳsi sarvāṇi ca yadvadanti ।
yadicchanto brahmacaryaṃ caranti
tatte padaꣳ saṅgraheṇa bravīmyomityetat ॥ 15॥

The seat and goal that all the Vedas glorify and which all austerities declare, for the desire of which men practise holy living, of That will I tell thee in brief compass. OM is that goal, O Nachiketas.