Musannaf Ibn Abi Shayba 36:324

Ubayd Allah ibn Musa narrated to us, he said: Musa ibn Ubaydah informed us, he said: Abu Murrah, the freed slave of Umm Hani, informed me, from Ibn Umar, who said: When the sacrificial animals were below the mountains that overlook the valley of al-Thaniyyah, the polytheists confronted him and turned back the faces of his sacrificial animals. So the Messenger of Allah (peace and blessings of Allah be upon him) slaughtered them where they had detained him, and that was al-Hudaybiyyah. He shaved his head, and some people followed his example and shaved. Others waited, saying: "Perhaps we will circumambulate the House." The Messenger of Allah (peace and blessings of Allah be upon him) said: "May Allah have mercy on those who shave." It was said: "And those who shorten?" He said: "May Allah have mercy on those who shave," three times.