Sahih Bukhari Volume 5, Book 59, Hadith Number 294.

Narrated By 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 they were over three-hundred-and-ten men.