Musannaf Ibn Abi Shayba 11:88

Yazid ibn Harun narrated to us, [saying] Yahya ibn Sa'id narrated to us, from Sa'id, from 'Abdullah ibn Abi Qatadah, from his father, who said: A man came to the Messenger of Allah, may Allah bless him and grant him peace, and said: "O Messenger of Allah, if I am killed in the way of Allah, will Allah expiate my sins for me?" The Messenger of Allah, may Allah bless him and grant him peace, said: "If you are killed in the way of Allah, while being patient, seeking reward, advancing, and not retreating, Allah will expiate your sins for you, except for debt. That is what Gabriel said to me."