Musnad Ahmad 937:210

Yahya ibn Sa’id narrated to us, from ‘Ikrimah ibn ‘Ammar, Abu Zumayl Simak al-Hanafi narrated to me, Malik ibn Murthad ibn ‘Abdullah al-Zamani narrated to me, my father Murthad narrated to me, he said: I asked Abu Dharr, I said: Did you ask the Messenger of Allah (peace and blessings of Allah be upon him) about the Night of Decree (Laylat al-Qadr)? He said: I was the one who asked the people about it. He said: I said: O Messenger of Allah, inform me about the Night of Decree: Is it in Ramadan or in other than it? He said: "Rather, it is in Ramadan." He said: I said: Does it exist with the prophets as long as they are present, and when they are taken (in death) it is lifted, or is it until the Day of Resurrection? He said: "Rather, it is until the Day of Resurrection." He said: I said: In which part of Ramadan is it? He said: "Seek it in the first ten and the last ten." Then the Messenger of Allah (peace and blessings of Allah be upon him) spoke and spoke, then I seized a moment of his distraction and said: In which of the two tens is it? He said: "Seek it in the last ten; do not ask me about anything after it." Then the Messenger of Allah (peace and blessings of Allah be upon him) spoke and spoke, then I seized a moment of his distraction and said: O Messenger of Allah, I adjure you by the right I have over you, that you inform me in which of the ten it is? He said: He became angry with me with an anger the likes of which he had not shown since I accompanied him—or he accompanied me—(a word similar to that). He said: "Seek it in the last seven; do not ask me about anything after it."