Musannaf Ibn Abi Shayba 21:106

Yahya ibn Adam narrated to us, from Hamza al-Zayyat, 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 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 received an excuse from me.'"