Reference : Hisn al-Muslim 127

اللّهُـمَّ أَنْتَ عَضُـدي، وَأَنْتَ نَصـيري،
بِكَ أَجـولُ وَبِكَ أَصـولُ وَبِكَ أُقـاتِل
Allāhumma anta `aḍudī, wa anta naṣīrī, 
bika ajūlu, wa bika aṣūlu, wa bika uqātil.


O Allah, You are my strength and You are my support. 
For Your sake, I go forth and for Your sake, I advance and for Your sake, I fight.

Reference:
Abu Dawud 3/42, At-Tirmidhi 5/572. See also Al-Albani, Sahih At-Tirmidhi 
3/183.

Chapter: Upon encountering an enemy or those of authority