Natyashastra
श्लिष्टा ललाटपट्टेष्वधोमुखी शम्भुरूपणे कर्या । शकस्याप्युत्ताना तज्ज्ञैस्तिर्यक्स्थिता कार्या ॥ ७७॥
To represent Śiva this hand pointing downwards is to be held close to the forehead and to indicate Śakra (Indra) this hand is to be raised ॥ to the forehead॥ and held across it.
english translation
zliSTA lalATapaTTeSvadhomukhI zambhurUpaNe karyA । zakasyApyuttAnA tajjJaistiryaksthitA kAryA ॥ 77॥
hk transliteration by Sanscript