Arabic text
عن أنس بن مالك رضي الله عنه قال: كان رسول الله صلى الله عليه وسلم إذا غزا، قال: «اللهم أنت عَضُدِي ونَصِيرِي، بِكَ أَحُول، وبِكَ أَصُول، وبك أقاتل».
Translation
Anas ibn Mālik (may Allah be pleased with him) reported: Whenever the Messenger of Allah (may Allah's peace and blessings be upon him) set out to participate in war, he would say: "O Allah, You are my Supporter and my Helper. With Your help I get strength, and with Your help I pounce upon the enemy and defeat it, and with Your help I fight."
Explanation
Whenever the Prophet (may Allah's peace and blessings be upon him) wished to launch a military campaign or set out on one, he would say what means: O Allah, You are my helper and supporter. It is by You alone that I carry out my deeds, and it is by You alone that I pounce upon the enemies of religion, and it is by You alone that I fight them.
Source: HadeethEnc.com