Śaunaka Śikṣā 8

8

catasro yādayo'ntasthā nāsikyā apyarephikāḥ
ūṣmāṇaśśādayaścaiva catvāraśceti kīrtitāḥ

The four beginning with 'ya' are semivowels (antasthā), and are also nasal, except for 'ra'. / The sibilants/spirants (ūṣman) beginning with 'śa' are proclaimed to be four.