Sahih Muslim 1857 a
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.
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 ‏.‏