Musnad Abu Ya'la al-Mawsili 111:106

Zuhayr narrated to us, Yazid bin Harun informed us, Muhammad bin Ishaq informed us, from Abdullah bin 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-Hudaybiyyah: "May Allah have mercy on those who shave." They said: "And those who shorten, O Messenger of Allah?" He said: "May Allah have mercy on those who shave." They said: "And those who shorten, O Messenger of Allah?" He said: "May Allah have mercy on those who shave." They said: "And those who shorten, O Messenger of Allah?" He said: "And those who shorten." They said: "O Messenger of Allah, what is the matter with those who shave, that you have singled them out with mercy?" He said: "Because they did not doubt."