Musannaf Ibn Abi Shayba 36:236

Zayd ibn al-Hubab narrated to us, from Musa ibn Ubaydah, who said: Muhammad ibn Thabit narrated to me that the Messenger of Allah (may Allah bless him and grant him peace) said on the day of Uhud: "Advance, Mus'ab." 'Abd al-Rahman said to him: "O Messenger of Allah, was Mus'ab not killed?" He said: "Yes, but an angel stood in his place and took his name."