Musnad Ahmad 658:88

Waki' narrated to us, my father and Sufyan and Isra'il narrated to us, from Abu Ishaq, from Al-Bara' ibn 'Azib, who said: We used to say that the number of the companions of the Messenger of Allah (peace and blessings of Allah be upon him) on the day of Badr was the same as the number of the companions of Talut (Saul) on the day of Jalut (Goliath): three hundred and a little over ten, those who crossed the river with him. He said: And none crossed the river with him except a believer.