Yājñavalkya Śikṣā 12

12

ṛco'rdhe tu dvimātraḥ syāttrimātraḥ syādṛgantake
riktaṃ tu pāṇimutkṣipya dvimātre dhārayedbudhaḥ

At the half of a verse, it should be two morae; at the end of a verse, it should be three morae. / Raising an empty hand, the wise person should hold it for two morae.