Musannaf Ibn Abi Shayba 36:327

Yazid ibn Harun narrated to us, he said: Muhammad ibn Ishaq informed us, from Ibn Abi Najih, from Mujahid, from Ibn Abbas, that the Messenger of Allah (may Allah bless him and grant him peace) said on the day of al-Hudaybiyah: "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," three times. They said: "And those who shorten, O Messenger of Allah?" He said: "And those who shorten." They said: "What is the matter with those who shave, that you have emphasized mercy for them?" He said: "Because they did not doubt."