It is narrated from Sayyiduna Sahl bin Sa'd (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "There are two times in which a supplication is not rejected: at the time of prayer, and at the time of battle."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 5826