Musannaf Ibn Abi Shayba 36:536

Ja'far ibn 'Awn narrated to us, from Muhammad ibn Sharik, from Ibn Abi Mulaykah, who said: 'Umar did not single out anyone from the people of the Shura (consultation council) over another, except that he secluded himself with 'Ali and 'Uthman, each one of them separately. He said: "O so-and-so, fear Allah. If Allah tests you with this matter, do not raise the children of so-and-so over the necks of the people." And he said the same to the other.