Shurayh bin Ubaid and Rashid bin Saeed and others say that during the journey to Syria, when Sayyiduna Umar (may Allah be pleased with him) reached a place called "Sargh," he received news that a severe plague had spread in Syria. Upon hearing this, he said: "I have received news that a plague has spread in Syria. My opinion is that if my last moment comes and Abu Ubaidah bin Al-Jarrah (may Allah be pleased with him) is alive, I will appoint him as my successor. And if Allah questions me about this, asking why I appointed him as the caliph over the Muslim Ummah, I will say that I heard Your Prophet say: 'Every prophet has a trustworthy one, and my trustworthy one is Abu Ubaidah bin Al-Jarrah.'" The people did not like this and began to say: "In that case, what will become of the great men of Quraysh, i.e., Banu Fihr?" Then Sayyiduna Umar (may Allah be pleased with him) said: "If Abu Ubaidah passes away before my death, I will appoint Mu'adh bin Jabal (may Allah be pleased with him) as my successor. And if Allah asks me why I appointed him as caliph, I will say that I heard Your Prophet say that he will be raised on the Day of Resurrection as a leader among the scholars."
Hadith Referenceمسند احمد / مسند الخلفاء الراشدين / 108
Hadith Gradingحکم دارالسلام:حسن لغيره، وهذا إسناد رجاله ثقات إلا أن شريح بن عبيد وراشد بن سعد لم يدركا عمر