It is narrated from Hazrat Anas (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: Whoever forgets to pray or sleeps (and misses it), its expiation is only that he should offer it when he remembers, because Allah Almighty says: "Establish prayer for My remembrance."
And he said that when the Prophet (peace be upon him) set out for any battle, he would say: O Allah! You are my strength, You are my Lord, and through You alone I fight.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 12909