مُسْنَدُ أَحْمَدَ
Musnad Ahmad 2665
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated that Ibn 'Abbas (رضي الله عنه) said: The Messenger of Allah (ﷺ) said: "The best number for companionship is four, the best number for an expedition is four hundred, the best number for an army is four thousand, and twelve hundred will never be defeated due to being too few in number.”
Musnad Ahmad 2682A mursal hadeeth]