Natyashastra
द्विर्भावाद्विकलस्यैवं विज्ञेयं च चतुष्कलम् । सप्तमे लघुनि त्वन्त्यो द्विर्भावोऽस्मिन्विधीयते ॥ २३१॥
By the repetition of the two-Kalā unit, the four-Kalā form is to be known. In this, the seventh short syllable is repeated at the end.
english translation
dvirbhAvAdvikalasyaivaM vijJeyaM ca catuSkalam । saptame laghuni tvantyo dvirbhAvo'sminvidhIyate ॥ 231॥
hk transliteration by Sanscript