Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said that the Messenger of Allah (peace and blessings be upon him) said: "The best of companions are four, the best expedition (sariyyah) is of four hundred men, the best army is of four thousand, and an army of twelve thousand will not be overcome due to small numbers."
Jarir bin Hazim alone narrated it with a connected chain, while Uthman bin Umar narrated it from Yunus, from Uqayl, from al-Zuhri, from the Prophet (peace and blessings be upon him) with a broken chain (munqati'). Abu Dawud said: Jarir bin Hazim gave it a full chain (musnad), and that is a mistake.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18481