Katha Upanishad Verse 2.1.15

True knowledge is the realization of unity. Just as pure water poured into pure water remains the same, the soul of a thinker who has realized the Truth becomes one with the Supreme Self.

Sanskrit Verse

यथोदकं शुद्धे शुद्धमासिक्तं तादृगेव भवति ।
एवं मुनेर्विजानत आत्मा भवति गौतम ॥ १५ ॥

Transliteration

yathodakaṃ śuddhe śuddhamāsiktaṃ tādṛgeva bhavati ।
evaṃ munervijānata ātmā bhavati gautama ॥ 15 ॥

Translation

As water pure poured into pure becomes the same only, so the atman of the thinker who knows thus, becomes; Oh Gautama.