The engraving on the ring of Abu Ubaydah (may Allah be pleased with him) was: loyalty is rare

It is narrated from Abdullah bin Wahb that the engraving on the ring of Sayyiduna Abu Ubaydah bin al-Jarrah (may Allah be pleased with him) was "al-Wafa'u aziz" (Loyalty is rare).

Silah bin Zufar narrates from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that the two chiefs of Najran, al-Aqib and al-Sayyid, came to the Prophet (peace and blessings be upon him) wanting to engage in mutual imprecation (mubahalah) with him. One of them said to his companion: "Do not do it, for by Allah, if he is a true prophet and he invokes a curse upon us, neither we nor our descendants after us will ever prosper." So the two of them said: "(Instead of mubahalah) we will give you the wealth that you ask for; send with us a man who is truly trustworthy." The narrator says: Hearing this, the Companions of the Messenger of Allah (peace and blessings be upon him) began to hope for this honour. He (peace and blessings be upon him) said: "Stand up, O Abu Ubaydah bin al-Jarrah." Then, when he was about to depart, the Messenger of Allah (peace and blessings be upon him) said: "This is the trustee of this Ummah."
The two Shaykhs agreed on recording this hadith in abridged form through the narration of Hudhayfah (may Allah be pleased with him), whereas Isra'il, differing from al-Thawri and Shu'bah, narrated it via Silah from Ibn Mas'ud, and its wording is more complete than the other chains. Therefore I have recorded it, because it is sahih (authentic) on the condition of both of them.