Sahih Muslim Book 7, Hadith Number 2985.

Chapter : It is preferable to get one’s hair cut (as a ritual of pilgrimage) but clipping is also permissible.

Ibn ‘Umar reported that Allah’s Messenger (may peace be upon him) said: May Allah have mercy upon those who have got their heads shaved. They said: Messenger of Allah, (what about) those who got their hair clipped? He said: May Allah have mercy upon those who have got their heads shaved. They said: Messenger of Allah, (what about those who have got their hair clipped)? He said: May Allah have mercy upon those who got their hair shaved. They said: Messenger of Allah, (what about) those who got their hair clipped? He said: (O Allah, have mercy upon) those who got their hair clipped.