Natyashastra
त्रिकलश्च सन्निपातो भूयो विधिरेष कर्तव्यः । द्वादश कलास्तृतीये तस्यान्ते सन्निपातश्च ॥ १५९॥
The Sannipāta should then consist of three Kalās, and this rule is to be followed again; in the third ॥ section॥ there should be twelve Kalās, ending with a Sannipāta.
english translation
trikalazca sannipAto bhUyo vidhireSa kartavyaH । dvAdaza kalAstRtIye tasyAnte sannipAtazca ॥ 159॥
hk transliteration by Sanscript