Musnad Ahmad 31:103

Muhammad ibn Abdullah ibn al-Zubayr narrated to us, Hisham—meaning Ibn Sa’d—narrated to us, from Zayd ibn Aslam, that Abdullah ibn Umar opened a small door of his, and Abu Sa’id al-Khudri was with him. A snake came out upon them, and Abdullah ibn Umar ordered its killing. Abu Sa’id said: "Do you not know that the Messenger of Allah, may Allah bless him and grant him peace, ordered that we warn them before we kill them?"