اُغْزُوا بِاسْمِ اللَّهِ فِي سَبِيْلِ اللَّهِ قَاتِلُوا مَنْ كَفَرَ بِاللَّهِ اُغْزُوا وَلَا تَغُلُّوا وَلَا تَغْدِرُوا وَلَا تَمْثُلُوا وَلَا تَقْتُلُوا وَلِيْدًا

Ughzu bismillaahi fi sabilillaahi qaatilu man kafara billaahi. Ughzu wa laa tagullu wa laa tagdiru wa laa tamthulu wa laa taqtulu walida.

Fight in the name of Allaah and in the path of Allaah and with him who disbelieves in Allaah fight and do not be treacherous and do not be dishonest about boot yand do not deface (in killing) and do not kill a child. If someone is made the amir (commander) of a small or large army (or on behalf of a contemporary ruler) he will bequeath to him the fear of Allah about himself and to treat the Muslims (Soldiers) under him well. Then say - (the Dua is mentioned above)

Reference: Muslim: 1731