Musnad Ahmad 27:224

Yahya narrated to us, from Ubaydullah, Nafi’ informed me, from Ibn Umar, that some of the companions of the Prophet, peace and blessings of Allah be upon him, saw the Night of Decree (Laylat al-Qadr) in a dream during the last seven nights. The Messenger of Allah, peace and blessings of Allah be upon him, said: "I see that your dreams have concurred regarding the last seven nights, so seek it in the last seven nights."