CHĀNDOGYA UPANISHAD · 1.1.3
स एष रसानां रसतमः परमः परार्ध्यो ऽष्टमो यदुद्गीथः ॥
sa eṣa rasānāṃ rasatamaḥ paramaḥ parārdhyo ’ṣṭamo yadudgīthaḥ ॥
That udgîtha (Om) is the best of all essences, the highest, deserving the highest place, the eighth.
F. Max Müller · The Upanishads, Sacred Books of the East 1 (1879)