Brahma Sutras
ॐ श्रुतेस्तु शब्दमूलत्वात् ॐ ॥ २-१-२७॥
But (this has to be accepted) on the authority of the Upanishad, for Brahman is known from the Upanishads alone.
english translation
OM zrutestu zabdamUlatvAt OM ॥ 2-1-27॥
hk transliteration by Sanscript