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Al-Adab al-Mufrad 661
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.
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."
Reported by Malik (No. 153). Cited in Al-Adab al-Mufrad (No. 661).