Yājñavalkya Śikṣā 15

15

praṇavaṃ tu plutaṃ kuryādvyāhṛtīrmmātrikā viduḥ
cāṣastu vadate mātrāṃ dvimātrāṃ vāyaso'bravīt

One should make the syllable Om protracted; they know the sacred utterances to have one mora. / The blue jay sounds one mora; the crow has spoken two morae;