Sahih Muslim Book 19, Hadith Number 4437.

Chapter : The battle of Khaibar.

It has been narrated on the authority of Anas that the Messenger of Allah (may peace be upon him) raided Khaibar. One morning we offered prayers in the darkness of early dawn (near Khaibar). Then the Messenger of Allah (may peace be upon him) mounted (his horse). Abu Talha mounted his and I mounted behind Abu Talha on the same horse. The Prophet of Allah (may peace be upon him) rode through the streets of Khaibar and (I rode so close to him) that my knee touched the thigh of the Prophet of Allah (may peace be upon him). The wrapper got aside from his thigh, and I could see its whiteness. When he entered the town, he said: God is Great. Khaibar shall face destruction. When we descend in the city-square of a people, it is a bad day for them who have been warned (and have not taken heed). He said these words thrice. The people of the town had just come out from (their houses) to go about their jobs. They said (in surprise): Muhammad has come. We captured Khaibar by force.