ENOfficial
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.
Source: sunnah.com
Other translations
Arabic
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.
sunnah.com