The Messenger of Allah (ﷺ), used to say during the battle:

اَللَّهُمَّ أَنْتَ عَضُدِي، وَأَنْتَ نَصِيرِي، بِكَ أَحُولُ وَبِكَ أَصُولُ، وَبِكَ أُقَاتِلُ

Allaahumma 'Anta 'adhudee, wa 'Anta naseeree, bika 'a'hoolu, wa bika 'asoolu, wa bika 'uqaatilu

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: Hasan Gharib. Abu Dawud: 2632