Musnad Ahmad 32:728

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from Qatadah, from Anas, who said: When the Prophet, peace and blessings of Allah be upon him, came to Khaybar, he found them when they had gone out to their crops, and with them were their hoes. When they saw him and the army with him, they retreated and returned to their fortress. The Prophet, peace and blessings of Allah be upon him, said: "Allah is the Greatest. Khaybar is ruined. Indeed, when we descend into the courtyard of a people, evil is the morning of those who have been warned."