Musnad Ahmad 26:815

Yunus narrated to us, Hammad—meaning Ibn Zayd—narrated to us, from ‘Asim, from Abu Wa’il, from ‘Abdullah ibn Mas‘ud, who said: When the Messenger of Allah (may Allah bless him and grant him peace) distributed the spoils of Hunayn at al-Ji‘ranah, they crowded around him. The Messenger of Allah (may Allah bless him and grant him peace) said: "A servant from among the servants of Allah was sent by Allah to his people, and they struck him and wounded him." He said: "So he began to wipe the blood from his forehead and say: 'O Lord, forgive my people, for they do not know.'" ‘Abdullah said: It is as if I am looking at the Messenger of Allah (may Allah bless him and grant him peace) wiping the blood from his forehead, imitating the man, and saying: "O Lord, forgive my people, for they do not know."