The works of Ādi Śaṅkarācārya

Chāndogyopaniṣadbhāṣyam

Verse 297

दध्नः सोम्य मथ्यमानस्य योऽणिमास ऊर्ध्वः समुदीषति तत्सर्पिर्भवति ॥ १ ॥

In plain words

When curd is churned, my dear, the subtlest part of it rises to the top and turns into butter.

Close to the Sanskrit

Of curd, my dear, being churned, whichever is the subtlety, that rising upward comes forth, that becomes butter.