Sahl ibn Sa‘d (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "Two supplications are never rejected, or are rarely rejected: supplication made upon hearing the call to prayer and during battle when the two armies are engaged in fighting.”
عن سهل بن سعد -رضي الله عنهما- مرفوعاً: «ثنتان لا تُرَدَّانِ، أو قلما تردان: الدعاء عند النداء وعند البأس حِينَ يُلْحِمُ بَعْضُهُ بَعْضًا».
شرح الحديث :
This Hadīth clarifies the virtue of Jihad, as Allah, the Exalted, answers the supplication of the Mujāhid when he is engaged in the battle. It also clarifies the virtue of the call to prayer as Allah, the Exalted, answers the supplication of the Muslim when the Adhān is announced and until the Iqāmah is announced.