Musnad Ahmad 27:1774

I read to Abd al-Rahman: Malik, and Ishaq ibn Isa narrated to us, Malik informed us, from Nafi, from Abdullah ibn Umar, that the Messenger of Allah, may Allah bless him and grant him peace, said: "O Allah, have mercy on those who shave." They said: "And those who shorten, O Messenger of Allah?" He said: "O Allah, forgive those who shave." They said: "And those who shorten, O Messenger of Allah?" He said: "And those who shorten."