Narrated Hudhaifa:

Al-'Aqib and Saiyid, the rulers of Najran, came to Allah's Apostle 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 and we do this Lian, neither we, nor our offspring after us will be successful." Then both of them said (to the Prophet ), "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 Apostle wished to be that one. Then the Prophet ﷺ said, "Get up, O Abu 'Ubaida bin Al-Jarrah." When he got up, Allah's Apostle ﷺ said, "This is the Trustworthy man of this (Muslim) nation."

حَدَّثَنِي عَبَّاسُ بْنُ الْحُسَيْنِ، حَدَّثَنَا يَحْيَى بْنُ آدَمَ، عَنْ إِسْرَائِيلَ، عَنْ أَبِي إِسْحَاقَ، عَنْ صِلَةَ بْنِ زُفَرَ، عَنْ حُذَيْفَةَ، قَالَ جَاءَ الْعَاقِبُ وَالسَّيِّدُ صَاحِبَا نَجْرَانَ إِلَى رَسُولِ اللَّهِ ﷺ يُرِيدَانِ أَنْ يُلاَعِنَاهُ، قَالَ فَقَالَ أَحَدُهُمَا لِصَاحِبِهِ لاَ تَفْعَلْ، فَوَاللَّهِ لَئِنْ كَانَ نَبِيًّا فَلاَعَنَّا، لاَ نُفْلِحُ نَحْنُ وَلاَ عَقِبُنَا مِنْ بَعْدِنَا. قَالاَ إِنَّا نُعْطِيكَ مَا سَأَلْتَنَا، وَابْعَثْ مَعَنَا رَجُلاً أَمِينًا، وَلاَ تَبْعَثْ مَعَنَا إِلاَّ أَمِينًا. فَقَالَ " لأَبْعَثَنَّ مَعَكُمْ رَجُلاً أَمِينًا حَقَّ أَمِينٍ ". فَاسْتَشْرَفَ لَهُ أَصْحَابُ رَسُولِ اللَّهِ ﷺ فَقَالَ " قُمْ يَا أَبَا عُبَيْدَةَ بْنَ الْجَرَّاحِ ". فَلَمَّا قَامَ قَالَ رَسُولُ اللَّهِ ﷺ " هَذَا أَمِينُ هَذِهِ الأُمَّةِ ".