Sahih Muslim
The Book Pertaining to the Merits of the Companions (Allah Be Pleased With Them) of the Holy Prophet (May Peace Be Upon Him) (Kitab Al-Fadail Al-Sahabah)
Book Number: 031 / Hadith Number: 5949
Hudhaifa reported that the people of Najran came to Allah's Messenger (may peace be upon him) and said: Allah's Messenger, send along with us a man of trust; whereupon he said: I would definitely send to you a man of trust, a man of trust in the true sense of the term. Thereupon his Companions looked up eagerly and he sent Abu Ubaida b. Jarrah.