It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that he said to Sayyiduna Umar (may Allah be pleased with him): I have heard people saying something, and I will not fall short in conveying it to you. People think that you are not appointing anyone as your successor? He lowered his head for a moment and then raised it and said: Allah Himself will protect His religion. I will not appoint anyone as my successor because the Prophet (peace and blessings of Allah be upon him) also did not appoint anyone as his successor, and if I do appoint someone as successor, then Sayyiduna Abu Bakr (may Allah be pleased with him) also did the same.
Hadith Referenceمسند احمد / مسند الخلفاء الراشدين / 332