Musnad Abu Ya'la al-Mawsili 2:66

Abu Khaythama narrated to us, Jarir narrated to us, from Husayn, from ‘Amr ibn Maymun, from ‘Umar ibn al-Khattab, who said: When he was struck, ‘Abdullah ibn ‘Umar said to him, “Will you not appoint a successor, O Commander of the Faithful?” He said, “I do not find anyone more entitled to this matter than these people whom the Messenger of Allah (may Allah bless him and grant him peace) died while he was pleased with them.” He then named ‘Ali, ‘Uthman, Talha, al-Zubayr, ‘Abd al-Rahman ibn ‘Awf, and Sa‘d ibn Abi Waqqas. He said, “Let ‘Abdullah ibn ‘Umar witness them, though he has no share in the matter. Whomever they appoint as successor, he is the Caliph after me. If it falls to Sa‘d, then let the Caliph after me seek his assistance, for I did not remove him due to weakness or treachery.”