BṚHADĀRAṆYAKA UPANISHAD · 5.14.8

एतद् ध वै तज् जनको वैदेहो बुडिलम् आश्वतराश्विम् उवाच -- यन् नु हो तद् गायत्रीविद् अब्रूथाः ।
अथ कथं हस्तीभूतो वहसीति ।
मुखं ह्य् अस्याः सम्राण् न विदां चकरेति होवाच ।
तस्या अग्निर् एव मुखम् ।
यदि ह वा अपि बह्व् इवाग्नाव् अभ्यादधति सर्वम् एव तत् संदहति ।
एवं हैवैवंविद् यद्य् अपि बह्व् इव पापं कुरुते सर्वम् एव तत् संप्साय शुद्धः पूतो ऽजरो ऽमृतः संभवति ॥

etad dha vai taj janako vaideho buḍilam āśvatarāśvim uvāca -- yan nu ho tad gāyatrīvid abrūthāḥ ।
atha kathaṃ hastībhūto vahasīti ।
mukhaṃ hy asyāḥ samrāṇ na vidāṃ cakareti hovāca ।
tasyā agnir eva mukham ।
yadi ha vā api bahv ivāgnāv abhyādadhati sarvam eva tat saṃdahati ।
evaṃ haivaivaṃvid yady api bahv iva pāpaṃ kurute sarvam eva tat saṃpsāya śuddhaḥ pūto ’jaro ’mṛtaḥ saṃbhavati ॥

And thus Ganaka Vaideha spoke on this point to Budila Âsvatarâsvi: 'How is it that thou who spokest thus as knowing the Gâyatrî, hast become an elephant and carriest me?' He answered: 'Your Majesty, I did not know its mouth. Agni, fire, is indeed its mouth; and if people pile even what seems much (wood) on the fire, it consumes it all. And thus a man who knows this, even if he commits what seems much evil, consumes it all and becomes pure, clean, and free from decay and death.'

F. Max Müller · The Upanishads, Sacred Books of the East 15 (1884)