Musannaf Ibn Abi Shayba 36:212

Abu Usama narrated to us, from Ibn 'Awn, from Umayr ibn Ishaq, that the people scattered away from the Prophet, may God bless him and grant him peace, on the day of Uhud, while Sa'd ibn Malik was shooting, and a youth was handing arrows to him. Whenever an arrow ran out, he would push an arrow to him, then say, "Shoot, Abu Ishaq." When it was later, they searched for the youth, but they could not find him.