Majma al-Zawaid — Hadith 12406

Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrates: The Prophet (peace and blessings be upon him), mentioning the Mahdi, said: "If his period is short, it will be seven years; otherwise eight years; otherwise nine years. And he will surely fill the earth with justice and fairness, just as it had been filled with oppression and injustice before."
Al-Bazzar recorded it; its narrators are thiqat (trustworthy), and there is some weakness in some of its narrators.