Natyashastra
सर्पशीर्षौ यदा हस्तौ भवेतां स्वस्तिकस्थितौ । मध्यप्रसारिताङ्गुष्ठौ ज्ञेयौ सूचीमुखौ तदा ॥ १९३॥
When the hands are in the shape of a serpent's head, with the middle thumb stretched out, they are known as Sūcīmukha hands, placed in the position of the Svastika.
english translation
sarpazIrSau yadA hastau bhavetAM svastikasthitau । madhyaprasAritAGguSThau jJeyau sUcImukhau tadA ॥ 193॥
hk transliteration by Sanscript