Śaiśirīya Śikṣā 5

5

śaiśiro'haṃ pravakṣyāmi śākhāyā lakṣaṇaṃ vidhim
triṣaṣṭiścatuḥṣaṣṭirvā varṇāḥ saṃbhavato matāḥ

I, Śaiśira, shall proclaim the defining rule of the branch. / By origin, sixty-three or sixty-four phonemes are recognized