Narrated Anas:

The Prophet set out for Khaibar and reached it at night. He used not to attack if he reached the people at night, till the day broke. So, when the day dawned, the Jews came out with their bags and spades. When they saw the Prophet; they said, "Muhammad and his army!" The Prophet said, Allahu--Akbar! (Allah is Greater) and Khaibar is ruined, for whenever we approach a nation (i.e. enemy to fight) then it will be a miserable morning for those who have been warned."