It was narrated that Anas said:

"The Messenger of Allah ﷺ 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 ﷺ يَوْمَ خَيْبَرَ صَلاَةَ الصُّبْحِ بِغَلَسٍ وَهُوَ قَرِيبٌ مِنْهُمْ فَأَغَارَ عَلَيْهِمْ وَقَالَ " اللَّهُ أَكْبَرُ خَرِبَتْ خَيْبَرُ - مَرَّتَيْنِ - إِنَّا إِذَا نَزَلْنَا بِسَاحَةِ قَوْمٍ فَسَاءَ صَبَاحُ الْمُنْذَرِينَ " .