Sahih Bukhari Volume 5, Book 64, Hadith 4199

حَدَّثَنَا عَبْدُ اللَّهِ بْنُ يُوسُفَ، أَخْبَرَنَا مَالِكٌ، عَنْ حُمَيْدٍ الطَّوِيلِ، عَنْ أَنَسٍ ـ رضى الله عنه أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم أَتَى خَيْبَرَ لَيْلاً، وَكَانَ إِذَا أَتَى قَوْمًا بِلَيْلٍ لَمْ يُغِرْ بِهِمْ حَتَّى يُصْبِحَ، فَلَمَّا أَصْبَحَ خَرَجَتِ الْيَهُودُ بِمَسَاحِيهِمْ وَمَكَاتِلِهِمْ، فَلَمَّا رَأَوْهُ قَالُوا مُحَمَّدٌ وَاللَّهِ، مُحَمَّدٌ وَالْخَمِيسُ، فَقَالَ النَّبِيُّ صلى الله عليه وسلم ‏"‏ خَرِبَتْ خَيْبَرُ، إِنَّا إِذَا نَزَلْنَا بِسَاحَةِ قَوْمٍ فَسَاءَ صَبَاحُ الْمُنْذَرِينَ ‏"‏‏

Narrated By Anas : Allah's Apostle reached Khaibar at night and it was his habit that, whenever he reached the enemy at night, he will not attack them till it was morning. When it was morning, the Jews came out with their spades and baskets, and when they saw him(i.e. the Prophet), they said, "Muhammad! By Allah! Muhammad and his army!" The Prophet said, "Khaibar is destroyed, for whenever we approach a (hostile) nation (to fight), then evil will be the morning for those who have been warned."