Natyashastra
या ऋचः पाणिका गाथाः सप्तरूपाङ्ग एव च । सप्तरूपप्रमाणं हि तद्ध्रुवेत्यभिसञ्ज्ञिमम् ॥ २॥
The Ṛk (ṛcaḥ), Pāṇikā and Gāthā, and the Seven traditional Types of songs which have seven different measures, are called Dhruvās.
english translation
yA RcaH pANikA gAthAH saptarUpAGga eva ca । saptarUpapramANaM hi taddhruvetyabhisaJjJimam ॥ 2॥
hk transliteration by Sanscript