A shaykh from the people of Kufah said: I heard Sayyiduna Ali (may Allah be pleased with him) say: "You have asked me about a man who says to another man: 'O disbeliever! O sinner! O donkey!' There is no hadd (prescribed punishment) in it; there is only a punishment from the ruler. So do not go back to saying it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحدود / 17150