Musnad Ahmad 27:1551

Hashim narrated to us, Abu Mu'awiyah—meaning Shayban—narrated to us, from Uthman ibn Abdullah, who said: A man came to Ibn Umar and said: "O Ibn Umar, I am going to ask you about something, will you tell me about it?" He said: "Yes." Uthman [ibn Affan] was mentioned, and Ibn Umar said: "As for his absence from Badr, it was because the daughter of the Messenger of Allah (peace and blessings of Allah be upon him) was his wife, and she was ill. The Prophet (peace and blessings of Allah be upon him) said to him: 'You have the reward of a man who witnessed Badr, and his share [of the spoils].'" "As for his absence from the Pledge of Ridwan, if there had been anyone in the valley of Makkah more honorable than Uthman, he would have sent him. So he sent Uthman, and the Pledge of Ridwan took place after Uthman had gone to Makkah. The Messenger of Allah (peace and blessings of Allah be upon him) said with his right hand: 'This is the hand of Uthman,' and he struck his other hand upon it, saying: 'This is for Uthman.'" Then Ibn Umar said to him: "Go now with these [answers]."