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 number of companions is four, the best sariyyah (detachment of an army) is four hundred men, the best army is four thousand, and an army of twelve thousand will not be defeated on account of small numbers." This chain is sahih (authentic) on the condition of Imam Bukhari and Imam Muslim, but the two Shaykhs did not record it because of the disagreement among the narrators who report it from al-Zuhri.