Musannaf Ibn Abi Shayba 37:567

Abu Mu'awiyah narrated to us, from al-A'mash, from Mundhir ibn Ya'la, who said: On the day they wanted to kill Uthman, Marwan sent to Ali, "Will you not come to this man and prevent them? For they will not conclude [their affair] without you." Ali said, "I will certainly come to them." Then Ibn al-Hanafiyyah took him by his shoulders and embraced him, saying, "O my father, where are you going? By Allah, they will only increase you in fear." So Ali sent his turban to them, forbidding them from him.