Kautilya's Arthashastra 13.2.23

13.2.23

maṅgalye vā hrade taṭākamadhye vā rātrau tejanatailābhyaktā nāgarūpiṇaḥ śaktimusalāny ayomayāni niṣpeṣayantas tathaiva brūyuḥ //

Spies, disguised as Nagas (snake-gods and with their body besmeared with burning oil (tejánataila), may stand in the centre of a sacred pool of water or of a lake at night, and sharpening their iron swords or spikes, may shout aloud as before.