Digital Corpus of Sanskrit: Jaiminīya-Upaniṣad-Brāhmaṇa — Reference = book.section.sentence. The Kena Upaniṣad stands within it (4.18–21).
Digital Corpus of Sanskrit (Oliver Hellwig, 2010–2021), CC BY 4.0 — https://github.com/OliverHellwig/sanskrit. Text as typed there from the printed edition; Devanagari generated mechanically from its IAST.