Hadith 31:58
Sahl ibn Sa'd said, "There are two hours in which the gates of heaven are opened. At those times very rarely is the supplication of someone who makes supplication rejected:
when the call to prayer has been given and when in battle ranks in the Cause of Allah."
حَدَّثَنَا إِسْمَاعِيلُ قَالَ: حَدَّثَنِي مَالِكٌ، عَنْ أَبِي حَازِمٍ، عَنْ سَهْلِ بْنِ سَعْدٍ قَالَ: سَاعَتَانِ تُفْتَحُ لَهُمَا أَبْوَابُ السَّمَاءِ، وَقَلَّ دَاعٍ تُرَدُّ عَلَيْهِ دَعْوَتُهُ: حِينَ يَحْضُرُ النِّدَاءُ، وَالصَّفُّ فِي سَبِيلِ اللهِ
Original Reference: Al-Adab Al-Mufrad 661