Musnad Abu Ya'la al-Mawsili 110:261

Zuhayr narrated to us, Wahb ibn Jarir narrated to us, my father narrated to us, he said: I heard Yunus ibn Yazid al-Ayli narrating from al-Zuhri, from Ubayd Allah ibn Abd Allah, from Ibn Abbas, from the Prophet, peace and blessings of Allah be upon him, who said: "The best of companions are four, and the best of raiding parties (saraya) are four hundred, and the best of armies are four thousand. And twelve thousand shall never be overcome due to a small number."