Musnad Ahmad 920:18

Yahya ibn Adam narrated to us, Hamzah ibn Habib al-Zayyat narrated to us, from Abu Ishaq, from Sa'id ibn Jubayr, from Ibn Abbas, from Ubayy ibn Ka'b, who said: When the Messenger of Allah (peace and blessings of Allah be upon him) would supplicate for someone, he would begin with himself. He mentioned Musa one day and said: "May the mercy of Allah be upon us and upon Musa. If he had been patient, Allah the Exalted would have related to us more of his story. But he said: 'If I ask you about anything after this, then do not keep me as a companion; you have already obtained an excuse from me' [Al-Kahf: 76]."