Musnad Ahmad 25:1452

Yazid narrated to us, he said: Muhammad—meaning Ibn Ishaq—said: Abdullah ibn Abi Najih narrated to me, from Mujahid, from Ibn Abbas, who said: Some men shaved [their heads] on the day of al-Hudaybiyah, and others shortened [their hair]. The Messenger of Allah (peace and blessings of Allah be upon him) said: "May Allah have mercy on those who shave." They said: "O Messenger of Allah, and those who shorten?" He said: "May Allah have mercy on those who shave." They said: "O Messenger of Allah, and those who shorten?" He said: "May Allah have mercy on those who shave." They said: "O Messenger of Allah, and those who shorten?" He said: "And those who shorten." They said: "What is the matter with those who shave, O Messenger of Allah, that the mercy was doubled for them?" He said: "They did not doubt." He said: Then the Messenger of Allah (peace and blessings of Allah be upon him) departed.