Natyashastra
त्र्! यश्रश्च चतुरश्रश्च तालः कार्यो ध्रुवात्मकः । षट्कलोऽष्टकलश्चैव यस्तु पूर्वं प्रकीर्तितः ॥ १५॥
The Tāla used in a Dhruvā should be Tryaśra or Caturasra, consisting of six or eight Kalās respectively, as has been stated earlier.
english translation
tr! yazrazca caturazrazca tAlaH kAryo dhruvAtmakaH । SaTkalo'STakalazcaiva yastu pUrvaM prakIrtitaH ॥ 15॥
hk transliteration by Sanscript