Musnad Ahmad 31:846

Rawh, Abd al-Samad, and Abu Amir narrated to us, they said: Hisham ibn Abi Abd Allah narrated to us, from Yahya ibn Abi Kathir, from Abu Ibrahim—Abu Amir said: from Abu Ibrahim al-Ansari—from Abu Sa’id al-Khudri, that the Messenger of Allah (may Allah bless him and grant him peace) and his companions shaved their heads in the year of al-Hudaybiyyah, except for Uthman ibn Affan and Abu Qatadah. The Messenger of Allah (may Allah bless him and grant him peace) sought forgiveness for those who shaved three times, and for those who shortened [their hair] once.