Mundaka Upanishad Verse 3.1.5

Sanskrit

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

Transliteration

satyena labhyastapasā hyeṣa ātmā samyagjñānena brahmacaryeṇa nityam | antaḥśarīre jyotirmayo hi śubhro yaṃ paśyanti yatayaḥ kṣīṇadoṣāḥ || 5 ||

Translation

This Atman within the body, resplendent and pure, can be reached by truth and tapas, by sound knowledge and by abstinence from sexual pleasures constantly practised; he is within the body, resplendent and pure; him, assiduous Sanyasins see, their faults removed.

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Reference: Mundaka Upanishad