Musnad Ahmad 31:224

Ibn Numayr narrated to us, Musa—meaning al-Juhani—narrated to us, he said: I heard Zayd al-Ammi, he said: Abu al-Siddiq al-Naji narrated to us, he said: I heard Abu Sa’id al-Khudri, he said: The Prophet, peace and blessings of Allah be upon him, said: "There will be from my Ummah the Mahdi. Whether his life is long or his life is short, he will live for seven years, or eight years, or nine years. He will fill the earth with equity and justice, and the earth will bring forth its vegetation, and the sky will rain its rain."