Sahih Muslim Book 7, Hadith Number 2984.

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

Abdullah b. Umar (Allah be pleased with him) reported Allah’s Messenger (may peace be upon him) as having observed: O Allah, have mercy upon those who get their heads shaved. They (the Companions) said: Messenger of Allah, (what about those) who have got their hair clipped? He said: O Allah, have mercy upon those who have got their heads shaved. They (again) said: Allah’s Messenger, (what about those) who have got their hair clipped? Thereupon he said: (O Allah, have mercy upon those) who have got their hair clipped.