Bukhari

Hadith Commentary:
He was outstanding in honesty and trustworthiness. Although all the Companions were also honest and trustworthy, his rank in this particular quality was exceedingly elevated, just as the rank of Uthman (radi Allahu anhu) was in modesty, and the rank of Ali (radi Allahu anhu) was in courage.
(radi Allahu anhum ajma‘een)

Hadith Commentary:
All the noble Companions (radi Allahu anhum ajma'in) of the Messenger of Allah (sallallahu alayhi wa sallam) were believers and trustworthy, but Abu Ubaydah (radi Allahu ta'ala anhu) excelled even further in honesty and trustworthiness, just as Uthman (radi Allahu ta'ala anhu) was foremost in modesty, and Ali (radi Allahu ta'ala anhu) was foremost in bravery.

The Messenger of Allah (sallallahu alayhi wa sallam) selected Abu Ubaydah ibn al-Jarrah (radi Allahu ta'ala anhu) for the education of the people of Najran. This means that a solitary report (khabar wahid) is authoritative, and denial of its authority is only from those whose hearts harbor doubts and suspicions regarding the religion of Islam.

Hadith Commentary:
The Prophet (sallallahu alayhi wa sallam) invited them to Islam and conveyed the message, but they did not accept. Eventually, he said, "Come, let us engage in mubāhala," meaning that both parties should together supplicate to Allah, saying: "O Allah! Whoever among us is upon falsehood, send Your punishment upon him."
They were not willing to engage in mubāhala either; rather, they agreed to a treaty on the condition that they would give one thousand sets of garments in the month of Rajab and one thousand sets in the month of Safar, and with each set, they would also give one uqiyyah of silver.
The Qur'anic verse was revealed concerning them.

Hadith Commentary:
1.
Imam Bukhari rahimahullah has narrated the hadith from Anas radi Allahu anhu because its context is related to the invocation of blessings (salat) as mentioned in the hadith narrated from Hudhayfah radi Allahu anhu. Ibn Ishaq has mentioned that the Messenger of Allah sallallahu alayhi wa sallam sent Ali radi Allahu anhu to the people of Najran so that he might collect from them the alms (sadaqat), jizyah, or other dues.
(Zad al-Ma'ad: 3/557)
It is possible that the Messenger of Allah sallallahu alayhi wa sallam first sent Abu Ubaydah ibn al-Jarrah radi Allahu anhu so that he might collect the compensation for reconciliation from them, and after that sent Ali radi Allahu anhu so that he might collect jizyah from those Christians who remained upon their religion, and collect alms (sadaqah) and zakat from those who had become Muslim.

2.
From these ahadith, it is understood that if the opposing party does not accept the true evidences but rather rejects them, then it is legislated (mashru') to engage in mutual invocation of Allah's curse (mubahalah) with them.
Ibn Abbas radi Allahu anhu also invited one of his opponents to mubahalah, and Imam Awza'i rahimahullah also encountered an occasion for mubahalah with a group.
Hafiz Ibn Hajar rahimahullah writes that the party upon falsehood who engages in mubahalah is seized by Allah's punishment within a year.
He further states:
A heretic once engaged in mubahalah with me, and he perished within two months.
(Fath al-Bari: 8/119)

Hadith Commentary: Benefits and Issues: Allah Almighty had endowed all human noble qualities among your companions, but in certain attributes, some were granted special distinction. Abu Ubaydah Amir ibn Abdullah ibn al-Jarrah was distinguished in the attribute of trustworthiness (amanah), but distinction in a single quality does not grant absolute superiority; rather, consideration is given to the totality of attributes, in which Abu Bakr radi Allahu anhu excelled over all others.