Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) said: I went to Sayyidah Hafsah (may Allah be pleased with her), and she said: "Do you know that your father (Sayyiduna Umar (may Allah be pleased with him)) does not intend to nominate anyone as successor?" I said: "Certainly not." She said: "That is what he is going to do." I swore that I would certainly speak to him about it. Then I went off on a journey or a military expedition and could not speak to him. When I returned, I met my honoured father and he began asking me questions. I said: "I heard the people saying something, and I swore that I would say it to you: they claim that you will not nominate (anyone as) successor, whereas I know that if you had a shepherd tending your goats and he came to you having left his flock, you would consider that he had let them go to waste; and tending people is an even weightier matter." He said: He agreed with what I said, paused for a while, then raised his head and said: "Allah Almighty Himself will protect His religion. If I do not nominate (anyone as) successor, then the Messenger of Allah (peace and blessings be upon him) did not do so either; and if I nominate (someone as) successor, then Sayyiduna Abu Bakr al-Siddiq (may Allah be pleased with him) did so too." Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) said: When he mentioned only the Messenger of Allah (peace and blessings be upon him) and Sayyiduna Abu Bakr al-Siddiq (may Allah be pleased with him), I understood that he would not hold anyone equal to the Messenger of Allah (peace and blessings be upon him) (that is, he would not prefer anyone's way over his), and that he would not nominate a successor.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قتال أهل البغي / 16572