Musannaf Ibn Abi Shayba 38:2

'Isa ibn Yunus narrated to us, from al-A'mash, from a man whom he named, who said: I witnessed the Day of the Camel. I would not enter the house of al-Walid without remembering the Day of the Camel and the striking of swords upon the helmets. He said: I used to see 'Ali charging and striking with his sword until it would bend, then he would return and say: "Do not blame me, but blame this [sword]." Then he would return and straighten it.