Sunan al-Shafi'i

It is narrated from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that the Messenger of Allah (ﷺ) said (on the occasion of Hajj): "O Allah! Have mercy on those who shave their heads." Those who had their hair cut short requested, "Pray for mercy for those who have their hair cut short as well, O Messenger of Allah!" The Prophet (ﷺ) again prayed for those who shaved their heads. When they repeated their request, the Messenger of Allah (ﷺ) (the third time) said: "And (have mercy) on those who have their hair cut short as well."