Natyashastra
द्विकलेन प्रकर्त्व्यं मिश्रेणैककलेन वा । प्ररोचकं च कर्तव्यं गुरुप्रायोक्षरान्वितम् ॥ ३१४॥
It should be performed with the Dvikala or with a mixed Ekakala time-measure, and the opening (prarocaka) should be composed mainly of heavy syllables (guru-prāya) in structure.
english translation
dvikalena prakartvyaM mizreNaikakalena vA । prarocakaM ca kartavyaM guruprAyokSarAnvitam ॥ 314॥
hk transliteration by Sanscript