Arthashastra
परस्पर अपाश्रयाश् च एषां जाति संघाश् छिद्रेषु प्रहरेयुः, गूढाश् च अग्नि रस शस्त्रेण ॥
or persons, pretending to be the supporters of a local enemy and who are of the same caste, may under favourable opportunities kill him; or spies under concealment may kill local enemies with fire, poison, and weapons.
english translation
paraspara apAzrayAz ca eSAM jAti saMghAz chidreSu prahareyuH, gUDhAz ca agni rasa zastreNa ॥
hk transliteration by Sanscript