Sunan an-Nasa'i, Volume 1, Book 6, Hadith 548 (DarusSalam) ⓘ

It was narrated that Anas said: "The Messenger of Allah prayed Fajr on the day of Khaibar during the time it was still dark, when he was near the enemy. Then he attacked them and said: 'Allahu Akbar! Khaibar is destroyed!' Twice. 'Then, when it descends in their courtyard, evil will be the morning for those who had been warned!'" [1] [1] As-Saffat 37:177

أَخْبَرَنَا إِسْحَاقُ بْنُ إِبْرَاهِيمَ، قَالَ أَنْبَأَنَا سُلَيْمَانُ بْنُ حَرْبٍ، قَالَ حَدَّثَنَا حَمَّادُ بْنُ زَيْدٍ، عَنْ ثَابِتٍ، عَنْ أَنَسٍ، قَالَ صَلَّى رَسُولُ اللَّهِ صلى الله عليه وسلم يَوْمَ خَيْبَرَ صَلاَةَ الصُّبْحِ بِغَلَسٍ وَهُوَ قَرِيبٌ مِنْهُمْ فَأَغَارَ عَلَيْهِمْ وَقَالَ ‏ "‏ اللَّهُ أَكْبَرُ خَرِبَتْ خَيْبَرُ - مَرَّتَيْنِ - إِنَّا إِذَا نَزَلْنَا بِسَاحَةِ قَوْمٍ فَسَاءَ صَبَاحُ الْمُنْذَرِينَ ‏" ‏ ‏.‏