It is narrated from Zayd bin Aslam (may Allah have mercy on him) that Sayyiduna Umar (may Allah be pleased with him) said to those six men, about whom he said that when the Messenger of Allah (peace and blessings be upon him) departed from this world he was pleased with them: "Whomever Abd al-Rahman bin Awf (may Allah be pleased with him) pledges allegiance to, you too pledge allegiance to him. Then, when you have pledged allegiance to the one to whom Abd al-Rahman (may Allah be pleased with him) has pledged allegiance, whoever refuses, strike his neck."
Hadith Referenceالمعجم الأوسط / باب الميم / 8076
Hadith Gradingمحدثین:إسناده ضعيف لانقطاعه، زيد لم يدرك عمر، وعبد الله بن زيد ضعفه الجمهور
Hadith Takhrij«قال الهيثمي: زيد لم يدرك عمر وولده عبد الله وثقه معن بن عيسى وغيره وضعفه الجمهور «مجمع الزوائد ومنبع الفوائد، ج 5، ص 185»»