Sahih al-Bukhari 3958
Narrated Al-Bara:
We, the Companions of Muhammad used to say that the number of the warriors of Badr was the same 
as the number of Saul's companions who crossed the river (of Jordan) with him, and none crossed the 
river with him but a believer, and the were over three-hundred-and-ten men.
حَدَّثَنَا عَبْدُ اللَّهِ بْنُ رَجَاءٍ، حَدَّثَنَا إِسْرَائِيلُ، عَنْ أَبِي إِسْحَاقَ، عَنِ الْبَرَاءِ، قَالَ كُنَّا أَصْحَابَ مُحَمَّدٍ صلى الله عليه وسلم نَتَحَدَّثُ أَنَّ عِدَّةَ أَصْحَابِ بَدْرٍ عَلَى عِدَّةِ أَصْحَابِ طَالُوتَ الَّذِينَ جَاوَزُوا مَعَهُ النَّهَرَ، وَلَمْ يُجَاوِزْ مَعَهُ إِلاَّ مُؤْمِنٌ، بِضْعَةَ عَشَرَ وَثَلاَثَمِائَةٍ‏.‏