Musnad Ahmad 27:1058

Rawh narrated to us, Malik narrated to us, from Nafi’, from 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, have mercy on those who shave." They said: "And those who shorten, O Messenger of Allah?" He said: "O Allah, have mercy on those who shave." They said: "O Messenger of Allah, and those who shorten?" He said: "And those who shorten."