Musannaf Ibn Abi Shayba 37:530

Abu Mu'awiyah and Ibn Numayr narrated to us, from Musa al-Juhani, from Zayd al-'Ammi, from Abu al-Siddiq al-Naji, from Abu Sa'id al-Khudri, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "There will be in my nation the Mahdi. If his life is long or his life is short, he will rule for seven years, or eight years, or nine years. He will fill it with equity and justice, just as it was filled with oppression and tyranny. The sky will pour down its rain, and the earth will bring forth its blessing. My nation will live in his time a life the likes of which they had never lived before that."