Sunan Ibn Majah, Volume 4, Book 24, Hadith 2827 (DarusSalam) ⓘ

It was narrated from Anas bin Malik that the Messenger of Allah said to Aktham bin Al-Jawn Al-Khuza’i: “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.”

حَدَّثَنَا هِشَامُ بْنُ عَمَّارٍ، حَدَّثَنَا عَبْدُ الْمَلِكِ بْنُ مُحَمَّدٍ الصَّنْعَانِيُّ، حَدَّثَنَا أَبُو سَلَمَةَ الْعَامِلِيُّ، عَنِ ابْنِ شِهَابٍ، عَنْ أَنَسِ بْنِ مَالِكٍ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ لأَكْثَمَ بْنِ الْجَوْنِ الْخُزَاعِيِّ ‏ "‏ يَا أَكْثَمُ اغْزُ مَعَ غَيْرِ قَوْمِكَ يَحْسُنْ خُلُقُكَ وَتَكْرُمْ عَلَى رُفَقَائِكَ يَا أَكْثَمُ خَيْرُ الرُّفَقَاءِ أَرْبَعَةٌ وَخَيْرُ السَّرَايَا أَرْبَعُمِائَةٍ وَخَيْرُ الْجُيُوشِ أَرْبَعَةُ آلاَفٍ وَلَنْ يُغْلَبَ اثْنَا عَشَرَ أَلْفًا مِنْ قِلَّةٍ ‏" ‏ ‏.‏