Hazrat Umar (radhiyallahu ‘anhu) was once seated with his son, Hazrat Abdullah bin Umar, his cousin, Hazrat Sa’eed bin Zaid, and Hazrat Abbaas (radhiyallahu ‘anhum). Hazrat Umar (radhiyallahu ‘anhu) mentioned to them, “I have decided that I will not appoint any specific person as the Khalifah after me.”
Hazrat Sa’eed bin Zaid (radhiyallahu ‘anhu), being concerned about the welfare of the Muslims and the affair of khilaafah, responded saying, “Perhaps, if you indicate towards some person from among the Muslims who is worthy for this position of khilaafah, then the people may be able to rely on your decision (and thus appoint that person as the khalifah after you).” He also mentioned to Hazrat Umar (radhiyallahu ‘anhu) that Hazrat Abu Bakr (radhiyallahu ‘anhu) appointed the khalifah before his demise and the people relied on his decision.
In reply to this, Hazrat Umar (radhiyallahu ‘anhu) said to him, “I have noticed that some people are wrongly hoping and desiring that they be appointed as the khalifah. Hence, I have decided to entrust this affair to six people and allow them to appoint one person from among them as the khalifah. The reason for me entrusting this matter to these six people is that they were the people whom Rasulullah (sallallahu alaihi wasallam) was most pleased with at the time of his demise (when he left the world).”
These six Sahaabah were: Hazrat Uthmaan, Hazrat Ali, Hazrat Zubair, Hazrat Sa’d bin Abi Waqqaas, Hazrat Talhah bin Ubaidullah and Hazrat Abdur Rahmaan bin Auf (radhiyallahu ‘anhum).
Hazrat Umar (radhiyallahu ‘anhu) then mentioned, “If one of two people was alive today and I had appointed any of them as the khalifah after me, then I would have had complete confidence (in him being worthy of the khilaafah). These two people are Saalim (radhiyallahu ‘anhu), the freed slave of Abu Huzaifah (radhiyallahu ‘anhu), and Abu Ubaidah bin Jarraah (radhiyallahu ‘anhu).”
Both these personalities had passed away during the khilaafah of Hazrat Umar (radhiyallahu ‘anhu). (Musnad Ahmad #129)