Musannaf Ibn Abi Shayba 36:187

'Abd al-Rahim ibn Sulayman narrated to us, from Zakariyya, from Abu Ishaq, from al-Bara' ibn 'Azib, who said: The companions of the Messenger of Allah (may Allah bless him and grant him peace) on the day of Badr were three hundred and some odd [men]. We used to say that they were the same number as the companions of Talut who crossed the river with him, and none crossed with him except a believer.