Musnad Ahmad 266:12

Muhammad ibn Abdullah ibn al-Zubayr narrated to us, he said: Abd al-Rahman ibn al-Ghasil informed us, from Abbas ibn Sahl, or Hamzah ibn Abi Usayd, from his father, who said: When we and the people met on the day of Badr, the Messenger of Allah (peace and blessings of Allah be upon him) said to us on that day: "If they overwhelm you—meaning, if they cover you—then shoot them with arrows." And I think he said: "And keep your arrows in reserve."