Mundaka 5.05

सत्येन लभ्यस्तपसा ह्येष आत्मा सम्यग् ज्ञानेन ब्रह्मचर्येण नित्यम्। अन्तःशरीरे ज्योतिर्मयो हि शुभ्रो यं पश्यन्ति यतयः क्षीणदोषाः ॥५॥

satyena labhyastapasā hyeṣa ātmā samyag jñānena brahmacaryeṇa nityam। antaḥśarīre jyotirmayo hi śubhro yaṃ paśyanti yatayaḥ kṣīṇadoṣāḥ ॥5॥

[सत्येन (satyena) - by truth; लभ्यः (labhyaḥ) - is obtained; तपसा (tapasā) - by austerity; हि (hi) - indeed; एषः (eṣaḥ) - this; आत्मा (ātmā) - Ātmān; सम्यक् (samyak) - properly; ज्ञानेन (jñānena) - by knowledge; ब्रह्मचर्येण (brahmacaryeṇa) - by celibacy; नित्यम् (nityam) - constantly; अन्तःशरीरे (antaḥśarīre) - within the body; ज्योतिर्मयः (jyotirmayaḥ) - full of light; हि (hi) - indeed; शुभ्रः (śubhraḥ) - pure; यम् (yam) - whom; पश्यन्ति (paśyanti) - see; यतयः (yatayaḥ) - ascetics; क्षीणदोषाः (kṣīṇadoṣāḥ) - whose faults are diminished;]

The Ātmān is realized through truthfulness, austerity, proper knowledge, and constant celibacy. Indeed, it is perceived within the body as full of light and pure by ascetics whose faults have diminished.