Hadith
حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مَسْلَمَةَ، عَنْ مَالِكٍ، عَنْ حُمَيْدٍ، عَنْ أَنَسٍ ـ رضى الله عنه أَنَّ النَّبِيَّ صلى الله عليه وسلم خَرَجَ إِلَى خَيْبَرَ فَجَاءَهَا لَيْلاً، وَكَانَ إِذَا جَاءَ قَوْمًا بِلَيْلٍ لاَ يُغِيرُ عَلَيْهِمْ حَتَّى يُصْبِحَ، فَلَمَّا أَصْبَحَ، خَرَجَتْ يَهُودُ بِمَسَاحِيهِمْ وَمَكَاتِلِهِمْ، فَلَمَّا رَأَوْهُ قَالُوا مُحَمَّدٌ وَاللَّهِ، مُحَمَّدٌ وَالْخَمِيسُ. فَقَالَ النَّبِيُّ صلى الله عليه وسلم " اللَّهُ أَكْبَرُ، خَرِبَتْ خَيْبَرُ، إِنَّا إِذَا نَزَلْنَا بِسَاحَةِ قَوْمٍ فَسَاءَ صَبَاحُ الْمُنْذَرِينَ ".
Narrated Anas:The Prophet (ﷺ) set out for Khaibar and reached it at night. He used not to attack if he reached the people at night, till the day broke. So, when the day dawned, the Jews came out with their bags and spades. When they saw the Prophet; they said, "Muhammad and his army!" The Prophet (ﷺ) said, Allahu--Akbar! (Allah is Greater) and Khaibar is ruined, for whenever we approach a nation (i.e. enemy to fight) then it will be a miserable morning for those who have been warned
Sahih
Sahih al-Bukhari 2945 ·Book 56, Hadith 157·Also available on sunnah.com·