Musannaf Ibn Abi Shayba 38:122

Waki’ narrated to us, he said: Al-A’mash narrated to us, from Shamir, from Abdullah bin Sinan al-Asadi, who said: I saw Ali on the day of Siffin, and with him was the sword of the Messenger of Allah, may Allah bless him and grant him peace, Dhu al-Fiqar. He said: He would strike with it, and it would slip, then he would charge at them. He said: Then he would come back. He said: Then he would charge at them. He said: Then he came with his sword, and it had become bent. He said: Indeed, this is apologizing to you.