ŚVETĀŚVATARA UPANISHAD · 1.16

सर्वव्यापिनम् आत्मानं क्षीरे सर्पिर् इवार्पितम् ।
आत्मविद्यातपोमूलं तद् ब्रह्मोपनिषत्परं तद् ब्रह्मोपनिषत्परम् ॥

sarvavyāpinam ātmānaṃ kṣīre sarpir ivārpitam ।
ātmavidyātapomūlaṃ tad brahmopaniṣatparaṃ tad brahmopaniṣatparam ॥

(If he looks) for the Self that pervades everything, as butter is contained in milk, and the roots whereof are self-knowledge and penance. That is the Brahman taught by the Upanishad.