CHĀNDOGYA UPANISHAD · 1.13.3
अनिरुक्तस् त्रयोदशः स्तोभः संचरो हुङ्कारः ॥
aniruktas trayodaśaḥ stobhaḥ saṃcaro huṅkāraḥ ॥
The thirteenth stobha syllable, viz. the indistinct syllable Huṅ, is the Undefinable (the Highest Brahman).
F. Max Müller · The Upanishads, Sacred Books of the East 1 (1879)