Sahl bin Sa'd (May Allah be pleased with him) reported:
The Messenger of Allah (ﷺ) said, "Supplications at two times are never turned down (or said, "Are seldom turned down"), a supplication after the Adhan has been proclaimed, and a supplication during the battle combating the enemy."[Abu Dawud].
App reference: Book 12, Hadith 41
Arabic/English book reference: Book 12, Hadith 1325