Muwatta Imam Malik (Riwayat Ibn al-Qasim)

And with this same chain (it is narrated from Sayyiduna Ibn Umar, may Allah be pleased with them both) that some people among the companions of the Messenger of Allah, peace and blessings be upon him, saw in their dreams that Laylat al-Qadr is in the last seven nights (of Ramadan). So the Messenger of Allah, peace and blessings be upon him, said: "I see that your dreams are in agreement with one another that Laylat al-Qadr is in the last seven nights, so whoever wants to seek it should seek it in the last seven nights."