Majma al-Zawaid 6878 — One who seizes land by force

Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) narrated: I said: "O Messenger of Allah, which wrongdoing is the greatest wrongdoing?" He said: "A cubit of land that a Muslim man seizes from the right of his brother; on the Day of Resurrection a (heavy) collar of it, down to the depths of the earth, will be put around his neck, and no one knows its depth except Allah Almighty, who created it."
Ahmad and al-Tabarani in al-Mu'jam al-Kabir recorded it; Ahmad's chain is hasan (good).