Natyashastra
अर्धत्रयोदशगणास्तृतीये वसुसञ्ज्ञिताः । एवमक्षरविन्यासो मध्यमासारिते स्मृतः ॥ १०५॥
In the Cañcatpuṭaḥ (lit. the first Tāla) there should be eight regular gaṇas, and the gaṇas in the Ṣaṭpitā-putrakaḥ should be twelve, and in the third part of the Vastu there should be thirteen gaṇas and a half. These is the arrangement of syllables in the medium Āsārita.
english translation
ardhatrayodazagaNAstRtIye vasusaJjJitAH । evamakSaravinyAso madhyamAsArite smRtaH ॥ 105॥
hk transliteration by Sanscript