Musnad Ahmad 1012:56

Yahya ibn Sa'id narrated to us, Humayd narrated to us, from Anas, from Ubadah ibn al-Samit, who said: The Messenger of Allah (peace and blessings of Allah be upon him) came out to us intending to inform us about the Night of Decree (Laylat al-Qadr). Two men argued, so it was lifted [from his memory]. He said: "I came out intending to inform you about the Night of Decree, but two men argued, so it was lifted. Seek it in the ninth, the seventh, and the fifth." Ubaydah narrated to us and said: "Seek it in the ninth that remains."