Natyashastra
तत्रोपवहनं कार्यं प्रथमे तु गुरुद्वये । गुर्वक्षरे तृतीये तु ततः स्यात्प्रत्युपोहनम् ॥ २१५॥
In that, the Upavahana is to be performed in the first two long syllables. In the third long syllable, the Pratyupohana should then be executed.
english translation
tatropavahanaM kAryaM prathame tu gurudvaye । gurvakSare tRtIye tu tataH syAtpratyupohanam ॥ 215॥
hk transliteration by Sanscript