Sahih al-Bukhari 4380
Military Expeditions led by the Prophet (pbuh) (Al-Maghaazi)
The story of the people of Najran (Christians)
Narrated Hudhaifa: Al-`Aqib and Saiyid, the rulers of Najran, came to Allah's Messenger (ﷺ) with the intention of doing Lian one of them said to the other, "Do not do (this Lian) for, by Allah, if he is a Prophet (ﷺ) and we do this Lian, neither we, nor our offspring after us will be successful." Then both of them said (to the Prophet (ﷺ) ), "We will give what you should ask but you should send a trustworthy man with us, and do not send any person with us but an honest one." The Prophet (ﷺ) said, "I will send an honest man who Is really trustworthy." Then every one of the companions of Allah's Messenger (ﷺ) wished to be that one. Then the Prophet (ﷺ) said, "Get up, O Abu 'Ubaida bin Al-Jarrah." When he got up, Allah's Messenger (ﷺ) said, "This is the Trustworthy man of this (Muslim) nation."