Musnad Ahmad 25:859

Yunus narrated to us, Habban ibn Ali narrated to us, Uqayl ibn Khalid narrated to us, from al-Zuhri, from Ubayd Allah ibn Abd Allah ibn Utbah, from Ibn Abbas, who said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "The best of companions are four, the best of detachments are four hundred, and the best of armies are four thousand." He said: And the Messenger of Allah, peace and blessings of Allah be upon him, said: "A people will never be overcome due to fewness if they reach twelve thousand."