Natyashastra
गुरुप्राया तु सा कार्या तथा चैवावपाणिका । स्थायिवर्णाश्रयोपेता कलाष्टकविनिर्मिता ॥ १०३॥
It should predominantly consist of guru (long syllables) and be devoid of avapāṇika (short syllables). It is characterized by the foundation of stable vowels and consonants and is composed of eight kalās (measures).
english translation
guruprAyA tu sA kAryA tathA caivAvapANikA । sthAyivarNAzrayopetA kalASTakavinirmitA ॥ 103॥
hk transliteration by Sanscript