Hisnul Muslim 126

Upon encountering an enemy or those of authority

اللَّهُمَّ إِنَّا نَجْعَلُكَ فِي نُحُورِهِمْ وَنَعُوذُ بِكَ مِنْ شُرُورِهِمْ

Allaahumma 'innaa naj'aluka fee nuhoorihim wa na'oothu bika min shuroorihim.

O Allah, we ask You to restrain them by their necks and we seek refuge in You from their evil.