It is narrated from Qudamah bin Maz'un (may Allah be pleased with him) that Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) caught up with Sayyiduna Uthman bin Maz'un (may Allah be pleased with him) while both of them were on their mounts at Thaniyyat al-Uthayah, a place in al-Arj. His mount pressed against Uthman's mount, while the mount of the Messenger of Allah (peace and blessings be upon him) had gone ahead of the caravan. Sayyiduna Uthman bin Maz'un (may Allah be pleased with him) said: You have hurt me, O lock of tribulation! When the mounts began to move swiftly, Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) came near him and said: May Allah forgive Abu al-Sa'ib! What is this name you have given me? He said: No, by Allah, I am not the one who gave this name; rather the Messenger of Allah (peace and blessings be upon him) named you so. There he ((peace and blessings be upon him)) is, at the head of the caravan, leading the people. One day you passed by us while we were sitting with the Messenger of Allah (peace and blessings be upon him), and he (peace and blessings be upon him) said: "This is the lock of tribulation" - and he (peace and blessings be upon him) pointed with his hand - "as long as he lives among you, a firmly locked door will remain between you and tribulation."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 8289