Śaunaka Śikṣā 6

6

vargāṇāṃ prathamā varṇā dvitīyā ūṣmasaṃjñitāḥ
hakāravarjyāścāghoṣāḥ śiṣṭaṃ ghoṣavaducyate

The first [and third] sounds of the series, and the second [and fourth] are termed sibilants/aspirates (ūṣman); / [the first two] excluding 'ha' are unvoiced (aghoṣa); the remainder is said to be voiced (ghoṣavat).