Musnad Ahmad 30:305

Abu Mu’awiyah and Ya’la narrated to us, they said: Al-A’mash narrated to us, from Abu Salih, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "How much of the month has passed?" We said: "Twenty-two have passed, and eight remain." The Messenger of Allah (peace and blessings of Allah be upon him) said: "No, rather twenty-two have passed from it, and seven remain. Seek it [the Night of Decree] tonight." Ya’la said in his narration: "The month is twenty-nine."