Silsilat al-Ahadith al-Sahihah

It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "The best companion is four, the best detachment is of four hundred people, the best army is of four thousand people, and an army of twelve thousand will not be defeated merely due to small numbers."