Musnad Abu Ya'la al-Mawsili 42:290

Abu Khaythama narrated to us, Yazid bin Harun narrated to us, Hisham al-Dastawa'i informed us, from Yahya bin Abi Kathir, from Abu Ibrahim al-Ansari, from Abu Sa'id al-Khudri: That the Prophet (peace be upon him) shaved [his head] on the day of al-Hudaybiyyah, and his companions [did so] as well, except for Abu Qatadah and Uthman. The Messenger of Allah (peace be upon him) said: "May Allah have mercy on those who shave." They said: "And those who shorten [their hair], O Messenger of Allah?" He said: "May Allah have mercy on those who shave." They said: "O Messenger of Allah, and those who shorten?" He said: "And those who shorten," on the third [time].