Musnad Ahmad 5:333

Abu al-Mughirah narrated to us, Safwan narrated to us, Shurayh—meaning Ibn Ubayd—narrated to me, he said: The people of al-Sham were mentioned in the presence of Ali ibn Abi Talib while he was in Iraq, and they said: "Curse them, O Commander of the Faithful." He said: "No, for I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'The Abdal (substitutes) are in al-Sham, and they are forty men. Every time a man among them dies, Allah replaces his place with another man. By them, rain is sent down, by them victory is granted over the enemies, and by them, punishment is averted from the people of al-Sham.'"