Sahih Bukhari
Military Expeditions led by the Prophet (pbuh) (Al-Maghaazi)
Volume: 005 / Book Number: 059 / Hadith Number: 296
Narrated Al-Bara:
We used to say that the warriors of Badr were over three-hundred-and-ten, as many as the Companions of Saul who crossed the river with him; and none crossed the river with him but a believer.