KAṬHA UPANISHAD · 2.1.15

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

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

But as pure water that is poured into pure water, even as it was such it remaineth, so is it with the soul of the thinker who knoweth God, O seed of Gotama.