Musnad Ahmad 27:210

Yahya narrated to us, from Ubayd Allah, Nafi’ informed me, from Ibn Umar, who said: 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." He said in the fourth [time]: "And those who shorten."