Musnad Ahmad 27:1545

Hashim narrated to us, Layth narrated to us, Nafi’ narrated to us, from Abdullah ibn Umar, that he said: The Messenger of Allah (may Allah bless him and grant him peace) shaved, and a group of his companions shaved, and some of them shortened [their hair]. The Messenger of Allah (may Allah bless him and grant him peace) said: "May Allah have mercy on those who shave," once or twice. Then he said: "And those who shorten."