Natyashastra
कलानां वृद्धिमासाद्य त्वक्षराणां तु वर्धनात् । लयस्य वर्धनाच्चापि वर्धमानकमुच्यते ॥ १४९॥
This is the rule about the Tāla in all the Āsāritas. The Vardhamānaka is so called because of a ॥ gradual॥ increase of Kalās due to ॥ gradual॥ increase (vardhana) of syllables, and because of an increase of the Laya (tempo) in its successive phases.
english translation
kalAnAM vRddhimAsAdya tvakSarANAM tu vardhanAt । layasya vardhanAccApi vardhamAnakamucyate ॥ 149॥
hk transliteration by Sanscript