Musnad Ahmad 27:52

Isma'il narrated to us, Ayyub informed us, from Nafi', from Ibn 'Umar, who said: The people used to see visions, and they would relate them to the Messenger of Allah, may Allah bless him and grant him peace. He said: "I see—or he said—I hear that your visions have concurred upon the last seven [nights]. So whoever among you is seeking it, let him seek it in the last seven [nights]."