Musnad Ahmad 26:307

Abu al-Nadr narrated to us, Abu Mu'awiyah—meaning Shayban—narrated to us, from Abu al-Ya'fur, from Abu al-Salt, from Abu Aqrab, who said: I went early in the morning to Ibn Mas'ud one morning in Ramadan, and I found him sitting above his house. We heard his voice, and he was saying: "Allah has spoken the truth, and His Messenger has conveyed [the message]." We said: "We heard you saying: 'Allah has spoken the truth, and His Messenger has conveyed [the message].'" He said: "Indeed, the Messenger of Allah (peace and blessings of Allah be upon him) said: 'Verily, the Night of Decree (Laylat al-Qadr) is in the middle of the last seven nights of Ramadan; the sun rises on the morning following it clear, having no rays.'" I looked at it, and I found it to be as the Messenger of Allah (peace and blessings of Allah be upon him) had said.