Natyashastra
इत्युक्तं मद्रकं पादैस्त्रयोदशभिरन्वितम् । गुर्वक्षराणि चत्वारि चत्वारि स्युर्लघूनि च ॥ २३५॥
Thus the Madraka is said to consist of thirteen Pātas. It includes four heavy syllables and four light syllables.
english translation
ityuktaM madrakaM pAdaistrayodazabhiranvitam । gurvakSarANi catvAri catvAri syurlaghUni ca ॥ 235॥
hk transliteration by Sanscript