Abd al-Hamid bin Abd al-Rahman bin Zayd bin al-Khattab (may Allah have mercy on him) was governor of Kufah in the time of Umar bin Abd al-Aziz (may Allah have mercy on him). He wrote to Umar bin Abd al-Aziz (may Allah have mercy on him): A man was reviling you in a market of Kufah called al-Kunasah, and the charge was proven against him, so I thought either to kill him, or cut out his tongue, or flog him, but then I thought I should refer the matter to you. He wrote in reply: "Had you killed him, or cut out his tongue, or flogged him, I would have given you the same punishment. So go to al-Kunasah and revile him in return, or forgive him, and that is what I love more. No one's blood becomes lawful because he reviled one of the Muslims; but whoever reviles the Prophet (peace and blessings be upon him), his blood is lawful."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قتال أهل البغي / 16766