Śatapatha Brāhmaṇa ("of a hundred paths") 4.1.3.17

4.1.3.17

tasmādetadṛṣīṇābhyanūktam | catvāri vākparimitā padāni tāni vidurbrāhmaṇā ye manīṣiṇaḥ guhā trīṇi nihitā neṅgayanti turīyaṃ vāco manuṣyā vadantīti

Wherefore it has been thus spoken by the Rishi (Rig-veda I, 164, 45):--'Four are the measured grades of speech; the Brâhmans that are wise know them: three, deposited in secret, move not; the fourth grade of speech men speak.'