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.‏