Musnad Ahmad 3:77

'Affan narrated to us, Hammad bin Salamah narrated to us, Abu Sinan informed us, from Yazid bin Mawhab: That 'Uthman said to Ibn 'Umar: "Judge between the people." He said: "I will not judge between two, nor will I lead two men in prayer. Have you not heard the Prophet, peace and blessings of Allah be upon him, say: 'Whoever seeks refuge with Allah has sought refuge with a refuge'?" 'Uthman said: "Yes." He said: "Then I seek refuge with Allah that you should employ me." So he excused him and said: "Do not tell anyone about this."