Śaunaka Śikṣā 10

10

hrasvo dīrghaḥ pluta iti svarāḥ kālena saṃjñitāḥ
sandhyakṣarāṇāṃ kālaśca dīrghaplutau na mātrikaḥ

Vowels are designated by time as short (hrasva), long (dīrgha), and protracted (pluta). / And the duration of diphthongs (sandhyakṣara) is long and protracted; there is none of one mora.