(6) باب عِدَّةِ أَصْحَابِ بَدْرٍ
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)
Reference: Sahih al-Bukhari 3957
In-book reference: Book 64, Hadith 9
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 5, Book 59, Hadith 293