Hadith: O Allah, have mercy on those who shaved their heads. They said: And those who shortened their hair, O Messenger of Allah? He said: O Allah, have mercy on those who shaved their heads. They said: And those who shortened their hair, O Messenger of Allah? He said: O Allah, have mercy on those who shaved their heads. They said: And those who shortened their hair, O Messenger of Allah? He said: And those who shortened their hair

عن عبد الله بن عُمر رضي الله عنهما عن رسول الله صلى الله عليه وسلم أنه قال: «اللَّهُمَّ ارحم الْمُحَلِّقِينَ. قالوا: والْمُقَصِّرِينَ يا رسول الله؟ قال: اللهم ارحم الْمُحَلِّقِينَ. قالوا: والْمُقَصِّرِينَ يا رسول الله؟ قال: اللهم ارحم الْمُحَلِّقِينَ. قالوا: والْمُقَصِّرِينَ يا رسول الله؟ قال: والْمُقَصِّرِينَ».
[صحيح] - [متفق عليه]

‘Abdullāh ibn ‘Umar (may Allah be pleased with him) reported: The Messenger of Allah (may Allah's peace and blessings be upon him) said: "O Allah, have mercy on those who shaved their heads." They said: "And those who shortened their hair, O Messenger of Allah?" He said: "O Allah, have mercy on those who shaved their heads." They said: "And those who shortened their hair, O Messenger of Allah?" He said: "O Allah, have mercy on those who shaved their heads." They said: "And those who shortened their hair, O Messenger of Allah?" He said: "And those who shortened their hair."
[Authentic hadith] - [Narrated by Bukhari & Muslim]

Explanation

Shaving the head or shortening the hair is one of the rites of Hajj and ‘Umrah. Shaving, however, is better because by shaving one’s head, one shows his deep humility and obedience to Allah, the Almighty. Hence, the Prophet (may Allah's peace and blessings be upon him) invoked Allah's mercy three times on those who shave their heads. When he was reminded of those who shorten their hair, he kept on praying for those who shave their heads. Only in the third or fourth time did he include those who shorten their hair. This implies that shaving is preferable for men. It especially applies to those who are not performing ‘Umrah with the intention of waiting for Hajj, so there is little time for the hair to grow and get shaved in Hajj. In this case, shortening is better, since it will be shaved later.