Musnad Ahmad 920:15

Abdullah narrated to us, Muhammad ibn Abdullah ibn Numayr narrated to us, Abu Dawud Umar ibn Sa’d narrated to us, from Yahya ibn Zakariya ibn Abi Za’idah, from Hamzah, from Abu Ishaq, from Sa’id ibn Jubayr, from Ibn Abbas, from Ubayy: That the Messenger of Allah (peace and blessings of Allah be upon him) recited: "If I ask you about anything after it, then do not keep me as a companion; you have already obtained an excuse from me." [Al-Kahf: 76]