37:177As-Saaffaat

"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!'" As-Saffat 37:177

"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.'"

Allah's Messenger (ﷺ) reached Khaibar at night and it was his habit that, whenever he reached the enemy at night, he will not attack them till it was morning. When it was morning, the Jews came out with their spades and baskets, and when they saw him(i.e. the Prophet (ﷺ) ), they said, "Muhammad! By Allah! Muhammad and his army!" The Prophet (ﷺ) said, "Khaibar is destroyed, for whenever we approach a (hostile) nation (to fight), then evil will be the morning for those who have been warned."

There is a day in a year when descends the pestilence; at the end of the hadith Laith said that the non Arabs save themselves from it in Kanun Awwal (this is the month of December).