Riyad as-Salihin 1326 — The Book of Jihad

Anas Narrated from:

Whenever the Messenger of Allah (ﷺ) set out to participate in Jihad, he would supplicate: "Allahumma Anta 'adudi wa nasiri, bika ahulu, wa bika asulu, wa bika uqatilu (O Allah, You are my Supporter and my Helper. With Your help I get strength, and with Your help I bounce upon the enemy and defeat it, and with Your help I fight)."[Abu Dawud and At- Tirmidhi].