Agni Purana 241.17

241.17

ābhyāṃ pārśvābhighātau tu vyākhyātau kūṭayodhane /
purastādviṣame deśe paścāddhanyāttu vegavān //

Flank attacks in deceptive warfare are explained by these two [methods]. / When the ground in front is uneven, the swift one should strike from behind;