Musannaf Ibn Abi Shayba 27:296

Ibn ‘Uyaynah narrated to us, from al-Zuhri, from ‘Ubayd Allah, from Ibn ‘Abbas, who said: ‘Umar said: "For me to be brought forward and have my neck struck is more beloved to me than that I should be brought forward [to lead] a people among whom is Abu Bakr."