Śaunaka Śikṣā 17

17

svarāṇāmūṣmaṇāñcaiva vivṛtaṃ karaṇaṃ smṛtam
saṃvṛtañcetyakārasya sarvavarṇo nigadyate

The articulation of the vowels and the spirants is remembered as open (vivṛta); / and of the sound 'a', it is closed (saṃvṛta); thus every sound is described.