Kautilya's Arthashastra 13.4.43

13.4.43

pratipannaṃ yathoktam abhyāghātasaṃkulaṃ darśayitvā rātrau durgān niṣkrāntaṃ ghātayet //

Or a pretending ally of the enemy may send information to the besieged: "I am going to strike the besieging camp at such a time and place; then you should also fight along with me." When the enemy does so, or when he comes out of his fort after witnessing the tumult and uproar of the besieging army in danger, he may be slain as before.