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
English reference: Vol. 1, Book 6, Hadith 548
Arabic reference: Book 6, Hadith 552