Musnad Ahmad

It is narrated from Sayyiduna Ibn Mas'ud (may Allah be pleased with him) that once two men named Aqib and Sayyid came from Najran. They came with the intention of having a mutual imprecation (mubahala) with the Prophet (peace and blessings be upon him). One of them said to the other, "Do not engage in mubahala (and mutual cursing) with him, because if he is truly a Prophet and he sends a curse upon us through mubahala, then neither we nor our descendants will ever succeed." So both of them came to the service of the Prophet (peace and blessings be upon him) and said, "We will not do mubahala with you. We are ready to give you whatever you demand, just send with us a trustworthy man." The Prophet (peace and blessings be upon him) said, "I will send with you such a trustworthy man who truly deserves to be called trustworthy." Upon hearing this, the noble Companions (may Allah be pleased with them) started raising their heads and looking (in anticipation). Then the Prophet (peace and blessings be upon him) said, "O Abu Ubaidah bin Al-Jarrah! Stand up." When both of them were about to leave, the Prophet (peace and blessings be upon him) said, "He is the trustworthy one of this Ummah."