كتاب المغازى
64
Military Expeditions led by the Prophet (pbuh) (Al-Maghaazi)
(6)
Chapter: The number of the warriors of Badr
(6)
باب عِدَّةِ أَصْحَابِ بَدْرٍ
Sahih al-Bukhari 3957

Narrated Al-Bara:

The companions of (the Prophet) Muhammad who took part in Badr, told me that their number was that of Saul's (i.e. Talut's) companions who crossed the river (of Jordan) with him and they were over three-hundred-and-ten men. By Allah, none crossed the river with him but a believer. (See Qur'an 2:249)

حَدَّثَنَا عَمْرُو بْنُ خَالِدٍ، حَدَّثَنَا زُهَيْرٌ، حَدَّثَنَا أَبُو إِسْحَاقَ، قَالَ سَمِعْتُ الْبَرَاءَ ـ رضى الله عنه ـ يَقُولُ حَدَّثَنِي أَصْحَابُ، مُحَمَّدٍ صلى الله عليه وسلم مِمَّنْ شَهِدَ بَدْرًا أَنَّهُمْ كَانُوا عِدَّةَ أَصْحَابِ طَالُوتَ الَّذِينَ جَازُوا مَعَهُ النَّهَرَ، بِضْعَةَ عَشَرَ وَثَلاَثَمِائَةٍ‏.‏ قَالَ الْبَرَاءُ لاَ وَاللَّهِ مَا جَاوَزَ مَعَهُ النَّهَرَ إِلاَّ مُؤْمِنٌ‏.‏
Reference : Sahih al-Bukhari 3957
In-book reference : Book 64, Hadith 9
USC-MSA web (English) reference : Vol. 5, Book 59, Hadith 293
  (deprecated numbering scheme)