Musannaf Ibn Abi Shayba 24:164

Yahya ibn Adam narrated to us, he said: Ibn 'Uyaynah narrated to us, from Isma'il ibn Abi Khalid, from Hakim ibn Jabir, he said: I heard Talhah ibn 'Ubayd Allah say on the day of the Camel: "We had been lenient regarding the affair of Uthman, so we find no alternative but to be extreme."