Sahih Muslim
The Book of Pilgrimage (Kitab Al-Hajj)
Book Number: 007 / Hadith Number: 2983
'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."