Sunan Ibn Majah, Chapter: The Chapters on Jihad

that the Messenger of Allah (saw) said to Aktham bin Al-Jawn Al-Khuzai: "O Aktham! Fight alongside people other than your own, it will improve your attitude and make you generous to your companions. O Aktham, the best number of companions is four, the best number of troops on an expedition is four hundred, the best number of an army is four thousand, and twelve thousand will never be overpowered because of their small number."