Musannaf Ibn Abi Shayba 27:403

Husayn ibn Ali narrated to us, from Za’idah, from Kulayb ibn Wa’il, from Habib ibn Abi Mulaykah, who said: A man asked Ibn Umar about Uthman. He said, "Did he witness Badr?" He said, "No." He said, "Did he witness the Pledge of Ridwan?" He said, "No." He said, "Did he turn away on the day the two hosts met?" He said, "Yes." Then the man went away. It was said to Ibn Umar, "This man claims that you have disparaged Uthman." He said, "Bring him back." So they brought him back to him. He said to him, "Did you understand what I said to you?" He said, "Yes." He said, "You asked me if Uthman witnessed Badr, and I told you no. But the Messenger of Allah (may Allah bless him and grant him peace) said, 'O Allah, Uthman is in Your need and the need of Your Messenger,' so he struck a share for him. And you asked me if he witnessed the Pledge of Ridwan, and I told you no. But the Messenger of Allah (may Allah bless him and grant him peace) sent him to the Confederates (al-Ahzab) so that they might make a truce with us and make peace with us, but they refused. And the Messenger of Allah (may Allah bless him and grant him peace) pledged on his behalf and said, 'O Allah, Uthman is in Your need and the need of Your Messenger,' then he wiped one of his hands over the other, so he pledged on his behalf. And you asked me if Uthman turned away on the day the two hosts met, and I said yes. But Allah said: 'Indeed, those of you who turned back on the day the two hosts met, it was only Satan who caused them to slip because of some what they had earned; but Allah has already forgiven them.' So go, and exert your effort as much as you can."