After embracing Islam, the people of Najraan came to Rasulullah (sallallahu ‘alaihi wasallam) and requested him to send to them a trustworthy person (who could teach them deen).
Rasululllah (sallallahu ‘alaihi wasallam) said to them:
لأبعثن إليكم رجلا أمينا حق أمين
“I will send to you a person who is trustworthy, indeed very trustworthy.”
At that point, the Sahaabah (radhiyallahu ‘anhum) who were present all desired to be the recipient of this glad tiding of Rasulullah (sallallahu ‘alaihi wasallam). Rasulullah (sallallahu ‘alaihi wasallam) then appointed Hazrat Abu Ubaidah (radhiyallahu ‘anhu) to go to the people of Najraan and teach them deen.
When Hazrat Abu Ubaidah (radhiyallahu ‘anhu) stood up and was about to leave, Rasulullah (sallallahu ‘alaihi wasallam) mentioned regarding him, “This person is the (special) trustee of this Ummah”. (Saheeh Bukhaari #3480- 3481)
Note: The quality of trustworthiness was the hallmark of all the Sahaabah. However, on account of this quality being most outstanding in the life of Hazrat Abu Ubaidah (radhiyallahu ‘anhu), Rasulullah (sallallahu ‘alaihi wasallam) granted him this special title.