Musnad Abu Ya'la al-Mawsili 115:67

Khallad ibn Aslam narrated to us, al-Nadr ibn Shumayl narrated to us, Shu'bah narrated to us, from Qatadah, from Anas, who said: When the Messenger of Allah (may Allah bless him and grant him peace) came to Khaybar, he said: "Indeed, when we descend into the courtyard of a people, then evil is the morning of those who have been warned."