The Book on Government
33/18. Chapter:
It is recommended for the army to swear allegiance to the ruler when intending to fight, and an account of Bay'at Ar-Ridwan beneath the tree
Sahih Muslim
حَدَّثَنَا عُبَيْدُ اللَّهِ بْنُ مُعَاذٍ، حَدَّثَنَا أَبِي، حَدَّثَنَا شُعْبَةُ، عَنْ عَمْرٍو، - يَعْنِي ابْنَ مُرَّةَ - حَدَّثَنِي عَبْدُ اللَّهِ بْنُ أَبِي أَوْفَى، قَالَ كَانَ أَصْحَابُ الشَّجَرَةِ أَلْفًا وَثَلاَثَمِائَةٍ وَكَانَتْ أَسْلَمُ ثُمُنَ الْمُهَاجِرِينَ .
It has been narrated on the authority of 'Abdullah b. Abu Aufa who said: The Companions of the Tree (i e. those who swore fealty under the tree) were one thousand and three hundred, and the people of Aslam tribe were one-eighth of the Muhajirs.
Reference : Sahih Muslim 1857aIn-book reference : Book 33, Hadith 112USC-MSA web (English) reference : Book 20, Hadith 4584