Brihat Parashara Hora Shastra
यदि केन्द्रे त्रिकोणे वा निवसेतां तमोग्रहौ । नाथेनात्यतरेणाढ्यौ दृष्टौ वा योगकारकौ ॥ १७॥
If Rahu and/or Ketu are in Kendr, receiving a Drishti from, or in association with the Lord of a Kon, or of a Kendr, it will become Yog Karak.
english translation
yadi kendre trikoNe vA nivasetAM tamograhau । nAthenAtyatareNADhyau dRSTau vA yogakArakau ॥ 17॥
hk transliteration by Sanscript