Umar bin Abd al-Aziz (may Allah have mercy on him) said that al-Walid bin Abd al-Malik sent for him (and asked): Should someone who reviles the caliphs be killed? I remained silent. He asked again, and again I remained silent. Then he asked: Why do you not speak? So I asked: O Commander of the Faithful, has he killed (anyone)? He said: No, he reviled (them). I said: Then let him be given an exemplary punishment for the sanctity of the caliphs that he has violated.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قتال أهل البغي / 16765