Natyashastra
संमुखप्रसृताङ्गुष्ठः कार्यो बालेन्दुदर्शने । पराङ्ग्मुखस्तु कर्तव्यो याने नृणां प्रयोक्तृभिः ॥ ३८॥
With the thumb extended forward, this hand should be used to represent the crescent moon. When turned away, it should be used by actors to depict a person's journey.
english translation
saMmukhaprasRtAGguSThaH kAryo bAlendudarzane । parAGgmukhastu kartavyo yAne nRNAM prayoktRbhiH ॥ 38॥
hk transliteration by Sanscript