Musnad Ahmad 27:1809

Ibn Numayr narrated to us, Ubaydullah narrated to us, from Nafi’, from Ibn Umar, that the Messenger of Allah (peace and blessings of Allah be upon him) 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." Then he said in the fourth [instance]: "And those who shorten."