Śaiśirīya Śikṣā 14

14

catasro yādayontasthā nāsikyā apyarephikāḥ
ūṣmāṇaḥ śādayaścaiva catvāra iti kīrtitāḥ

There are four semivowels beginning with ya, and [they can be] nasal, except for r. / And the spirants beginning with śa are proclaimed to be four.