Narrated Anas: "When the Messenger of Allah (ﷺ) set out for Khaibar, he approached it at night and when he came to a people during the night, he would not attack them until morning. So when the morning came, the Jews came out with their shovels and baskets, then when they saw him, they said: 'Muhammad! By Allah Muhammad has come with the Khamis (an army).' So the Messenger of Allah (ﷺ) said: 'Allahu Akbar! Khaibar is destroyed, for whenever we approach the land of people - then what an evil morning for those who have been warned.'"
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا الأَنْصَارِيُّ، حَدَّثَنَا مَعْنٌ، حَدَّثَنِي مَالِكُ بْنُ أَنَسٍ، عَنْ حُمَيْدٍ، عَنْ أَنَسٍ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم حِينَ خَرَجَ إِلَى خَيْبَرَ أَتَاهَا لَيْلاً وَكَانَ إِذَا جَاءَ قَوْمًا بِلَيْلٍ لَمْ يُغِرْ عَلَيْهِمْ حَتَّى يُصْبِحَ فَلَمَّا أَصْبَحَ خَرَجَتْ يَهُودُ بِمَسَاحِيهِمْ وَمَكَاتِلِهِمْ فَلَمَّا رَأَوْهُ قَالُوا مُحَمَّدٌ وَافَقَ وَاللَّهِ مُحَمَّدٌ الْخَمِيسَ . فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " اللَّهُ أَكْبَرُ خَرِبَتْ خَيْبَرُ إِنَّا إِذَا نَزَلْنَا بِسَاحَةِ قَوْمٍ فَسَاءَ صَبَاحُ الْمُنْذَرِينَ " .
sunnah.com