Sahih Muslim Book 007, Hadith Number 2983.
Chapter : It is preferable to get one's hair cut (as a ritual of pilgrimage) but clipping is also permissible.
'Abdullah reported that Allah's Messenger (may peace be upon him) got his head shaved (after slaughtering the sacrificial animal on the 10th of Dhu'l-Hijja), and so did a group of Companions, while some of them got their hair clipped. Abdullah said: Allah's Messenger (may peace be upon him) observed once or twice: "May Allah have mercy upon those who get their heads shaved." And he also said: "Upon those too who got their hair clipped."