Chāndogya Upaniṣad 2.23.3

2.23.3

tāny abhyatapat \
tebhyo 'bhitaptebhya oṃkāraḥ saṃprāsravat \
tad yathā śaṅkunā sarvāṇi parṇāni saṃtṛṇṇāny evam oṃkāreṇa sarvā vāk saṃtṛṇṇā \
oṃkāra evedaṃ sarvam oṃkāra eva idaṃ sarvam \\223.3\

3 (4). He brooded upon them. From them, when they had been brooded upon, issued forth the syllable Om. As all leaves are held together by a spike, so all speech is held together by Om. Verily, Om is the world-all. Verily, Om is this world-all.