Mishkat al-Masabih 672
Sahl b. Sa'd reported God’s Messenger as saying, “Two things are not rejected, or are seldom rejected:
a supplication when the call to prayer is made, and in stress when people are locked in battle.” A version has “and when rain is falling”. 

Abu Dawud and Darimi transmitted it, but Darimi did not mention “and when rain is falling”.
وَعَنْ سَهْلِ بْنِ سَعْدٍ قَالَ قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «ثِنْتَانِ لَا تُرَدَّانِ أَوْ قَلَّمَا تُرَدَّانِ الدُّعَاءُ عِنْدَ النِّدَاءِ وَعِنْدَ الْبَأْسِ حِينَ يُلْحِمُ بَعْضُهُمْ بَعْضًا» وَفِي رِوَايَةٍ: «وَتَحْتَ الْمَطَرِ» . رَوَاهُ أَبُو دَاوُدَ وَالدَّارِمِيُّ إِلَّا أَنَّهُ لَمْ يَذْكُرْ «وَتَحْت الْمَطَر»