Musnad Abu Ya'la al-Mawsili 115:202

Narrated to us Muhammad ibn Mahdi, narrated to us Abd al-Razzaq, informed us Ma'mar, from Qatadah, from Anas, regarding His saying: "Then evil is the morning of those who were warned" (As-Saffat: 177). He said: When the Messenger of Allah (peace be upon him) came to Khaybar, he found them when they had gone out to their crops with their shovels. When they saw him and the army with him, they retreated and returned to their fortress. The Prophet (peace be upon him) said: "Allah is the Greatest! Khaybar is ruined. Indeed, when we descend into the courtyard of a people, then evil is the morning of those who were warned."