Bhagavatapurana 8.7.30

8.7.30

mukhāni pañcopaniṣadastaveśa yaistriṃśadaṣṭottaramantravargaḥ /
yat tac chivākhyaṃ paramātmatattvaṃ deva svayaṃjyotiravasthitiste //

29 O Lord, the five Upanishads are Thy faces, from which the thirty-eight mantras evolve; that highest truth called Siva, the self-effulgent form is, O god, Thy absolute state.