Musnad Ahmad 518:1

Surayj ibn al-Nu‘man narrated to us, Aws ibn ‘Abd Allah Abu Muqatil al-Saluli narrated to me, he said: Burayd ibn Abi Maryam narrated to me, from his father Malik ibn Rabi‘ah, that he heard the Messenger of Allah, may Allah bless him and grant him peace, saying: "O Allah, forgive those who shave. O Allah, forgive those who shave." A man from the people said: "And those who shorten?" The Messenger of Allah, may Allah bless him and grant him peace, said on the third or fourth time: "And those who shorten." He then said: "And I was on that day one whose head was shaved, and it would not please me to have had red camels or something of great value in exchange for the shaving of my head."