Musannaf Ibn Abi Shayba 3:6426

Azhar narrated to us, from Ibn ‘Awn, from Muhammad, who said: When Hujr was taken to Mu’awiyah, he said: "Peace be upon you, O Commander of the Faithful." He (Mu’awiyah) said: "And am I the Commander of the Faithful?" He said: "Yes." He said: "I will surely kill you." He said: Then he ordered that he be killed. He said: "Let me pray two rak'ahs." So he prayed two rak'ahs, making them brief. Then he said: "Do not think that I made them brief out of fear, but I disliked to prolong them for you." Then he was killed.