Musnad Ahmad 25:823

Wahb ibn Jarir narrated to us, my father narrated to us, he said: I heard Yunus narrating from al-Zuhri, from Ubayd Allah, from Ibn Abbas, he 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. And twelve thousand shall not be overcome due to a small number."