Arthashastra
दूष्येषु वा दूष्य व्यञ्जनाः प्रणिहिता दूष्यान् बाह्यैर् भेदयेयुः, बाह्यान् वा दूष्यैः ॥
Spies under the garb of traitors may mix with traitors and separate them from foreigners, or foreigners from local traitors.
english translation
dUSyeSu vA dUSya vyaJjanAH praNihitA dUSyAn bAhyair bhedayeyuH, bAhyAn vA dUSyaiH ॥
hk transliteration by Sanscript