Sahih Bukhari Volume 1, Book 10, Hadith 610

حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، قَالَ حَدَّثَنَا إِسْمَاعِيلُ بْنُ جَعْفَرٍ، عَنْ حُمَيْدٍ، عَنْ أَنَسِ بْنِ مَالِكٍ، أَنَّ النَّبِيَّ صلى الله عليه وسلم كَانَ إِذَا غَزَا بِنَا قَوْمًا لَمْ يَكُنْ يَغْزُو بِنَا حَتَّى يُصْبِحَ وَيَنْظُرَ، فَإِنْ سَمِعَ أَذَانًا كَفَّ عَنْهُمْ، وَإِنْ لَمْ يَسْمَعْ أَذَانًا أَغَارَ عَلَيْهِمْ، قَالَ فَخَرَجْنَا إِلَى خَيْبَرَ فَانْتَهَيْنَا إِلَيْهِمْ لَيْلاً، فَلَمَّا أَصْبَحَ وَلَمْ يَسْمَعْ أَذَانًا رَكِبَ وَرَكِبْتُ خَلْفَ أَبِي طَلْحَةَ، وَإِنَّ قَدَمِي لَتَمَسُّ قَدَمَ النَّبِيِّ صلى الله عليه وسلم‏.‏ قَالَ فَخَرَجُوا إِلَيْنَا بِمَكَاتِلِهِمْ وَمَسَاحِيهِمْ فَلَمَّا رَأَوُا النَّبِيَّ صلى الله عليه وسلم قَالُوا مُحَمَّدٌ وَاللَّهِ، مُحَمَّدٌ وَالْخَمِيسُ‏.‏ قَالَ فَلَمَّا رَآهُمْ رَسُولُ اللَّهِ صلى الله عليه وسلم قَالَ ‏"‏ اللَّهُ أَكْبَرُ، اللَّهُ أَكْبَرُ، خَرِبَتْ خَيْبَرُ، إِنَّا إِذَا نَزَلْنَا بِسَاحَةِ قَوْمٍ فَسَاءَ صَبَاحُ الْمُنْذَرِينَ ‏"

Narrated By Humaid : Anas bin Malik said, "Whenever the Prophet went out with us to fight (in Allah's cause) against any nation, he never allowed us to attack till morning and he would wait and see: if he heard Adhan he would postpone the attack and if he did not hear Adhan he would attack them." Anas added, "We reached Khaibar at night and in the morning when he did not hear the Adhan for the prayer, he (the Prophet) rode and I rode behind Abi Talha and my foot was touching that of the Prophet. The inhabitants of Khaibar came out with their baskets and spades and when they saw the Prophet they shouted 'Muhammad! By Allah, Muhammad and his army.' When Allah's Apostle saw them, he said, "Allahu-Akbar! Allahu-Akbar! Khaibar is ruined. Whenever we approach a (hostile) nation (to fight), then evil will be the morning of those who have been warned."