Yājñavalkya Śikṣā 13

13

ekamātro bhaveddhrasvo dvimātro dīrgha ucyate
trimātrastu pluto jñeyo vyañjanaṃ cārdhamātrikam

A short [vowel] has one mora; a long [vowel] is said to have two morae; / but a protracted [vowel] should be known as having three morae, and a consonant has a half-mora.