Musnad Ahmad 26:507

Bahz narrated to us, Hammad bin Zayd narrated to us, ‘Asim bin Bahdalah narrated to us, from Abu Wa’il, from ‘Abdullah bin Mas’ud, who said: The Messenger of Allah (may Allah bless him and grant him peace) distributed the spoils of Hunayn at al-Ji’ranah. He said: So they crowded around him. He said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Indeed, a servant from the servants of Allah was sent by Allah (the Almighty and Majestic) to his people, but they belied him and wounded him. He began wiping the blood from his forehead and saying: 'My 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 his forehead, imitating the man.