Natyashastra
अस्यैव तु यदा मुष्टेरूर्ध्वोङ्गुष्ठः प्रयुज्यते । हस्तः स शिखरो नाम तदा ज्ञेयः प्रयोक्तृभिः ॥ ५७॥
When the thumb of the Muṣṭi (fist) hand is raised upwards, it is known as the Śikhara hand gesture, as recognized by performers.
english translation
asyaiva tu yadA muSTerUrdhvoGguSThaH prayujyate । hastaH sa zikharo nAma tadA jJeyaH prayoktRbhiH ॥ 57॥
hk transliteration by Sanscript