Sayyiduna Sahl bin Sa'd (may Allah be pleased with him) reported that the Messenger of Allah (peace and blessings be upon him) said: "Two things are not rejected," or he said: "Two things are very rarely rejected: supplication at the time of the adhan, and at the time of battle when people are striking one another."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1938