Musnad Ahmad 435:60

Abdullah said: Ismail Abu Ma'mar and Muhammad ibn 'Abbad narrated to us, they said: Ibn 'Uyaynah narrated to us, from Hisham ibn Hujayr, from Tawus, who said: Mu'awiyah said to Ibn 'Abbas: "Do you not know that I shortened the hair of the Messenger of Allah, may Allah bless him and grant him peace, with a blade?" Ibn 'Abbas said: "No." Ibn 'Abbad said in his narration: Ibn 'Abbas said: "And this is an argument against Mu'awiyah."