Śaiśirīya Śikṣā 12

12

vargāṇāṃ prathamā varṇā dvitīyā ūṣmasaṃjñitāḥ
hakāravarjyāścāghoṣāḥ śiṣṭaṃ ghoṣavaducyate

The first phonemes of the classes [and] the second are designated as spirants; / and except for the sound ha, [they] are unvoiced; the rest is called voiced.