Atharvaveda 9.10.27

9.10.27

catvā́ri vā́k párimitā padā́ni tā́ni vidur brāhmaṇā́ yé manīṣíṇaḥ |
gúhā trī́ṇi níhitā néṅgayanti turī́yaṃ vācó manuṣyā̀ vadanti ||27||

Speech [is] four measured out quarters (padá); those are known by Brahmans who are skilful; three, deposited in secret, they do not set in motion (in̄gay-); a fourth of speech human beings speak.