From Sahih Bukhari (Hadith 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 they were over three-hundred-and-ten men
Reference: Bukhari 3958