Messenger of Allah ﷺ 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 ﷺ ), they said, "Muhammad! By Allah! Muhammad and his army!" The Prophet ﷺ 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."
أَنَّ رَسُولَ اللَّهِ ﷺ 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 ﷺ خَرِبَتْ خَيْبَرُ إِنَّا إِذَا نَزَلْنَا بِسَاحَةِ قَوْمٍ فَسَاءَ صَبَاحُ الْمُنْذَرِينَ