Musnad Ahmad 31:161

Yazid narrated to us, Hisham informed us, from Yahya, from Abu Ibrahim, from Abu Sa’id al-Khudri: "The Prophet, peace and blessings of Allah be upon him, entered into Ihram, and his companions, in the year of al-Hudaybiyyah, except for Uthman and Abu Qatadah. He sought forgiveness for those who shaved their heads three times, and for those who shortened their hair once."