Among his outstanding traits (radhiallahu ‘anhu) is what Al-Bukhari and Muslim collected on the authority of Anas bin Malik (radhiallahu ‘anhu) that Allah’s Messenger (sallallahu ‘alayhi wa sallam) said to Ubay,
إِنَّ اللهَ أَمَـرَنِـي أَنْ أَقْـرَأَ عَـلَـيْـكَ , قَـالَ : آلله سَـمَّـانِـي لَـكَ ؟ قَـالَ : اللهُ سَـمَّـاكَ لِـي . قَـالَ : فَـجَـعَـلَ أُبَـيٌّ يَـبْـكِـي .
“Indeed Allah ordered me to teach you” He (i.e. Ubay) said, “Did Allah mention my name to you?” the Prophet said, “Allah mentioned your name to me” Anas bin Malik said, “Ubay began to weep.”
Likewise you should contemplate in order to grasp Ubay’s the full extent of understanding of the religion (radhiallahu ‘anhu); his response to this question wasn’t after long period of time like a week or a month in which he could review the verses and contemplate its meanings; rather he (radhiallahu ‘anhu) merely responded immediately after the Messenger (sallallahu ‘alayhi wa sallam) repeated the question; so he chose this blessed verse.
This verse contains a concise lesson, a beneficial statement, and clarification of the three categories of at-Tawheed; it amasses the statement and clarification of at-Tawheed which is not found in any single verse; rather it is found dispersed in many verses.
Shaykh ‘Abdur Rahman As-Sa’dee said,
“This verse includes at-Tawheed as it relates to Allah’s divinity, lordship, and His names & attributes; in addition to that it includes the completeness of His dominion and what His knowledge encompasses, the vastness of His Power, Majesty, Greatness, Loftiness, and Exaltedness over all of His creation. This verse alone contains the ‘aqeedah as it relates to Allah’s Names and Attributes which comprises all of His most beautiful names and lofty attributes.” 
Ubay’s keenness (radhiallahu ‘anhu) in selecting this verse was profound and precise; and it demonstrates the great concern for at-Tawheed in the hearts of the companions. This is exactly what Al-Bukhari reported on the authority of ‘Ayesha (radhiallahu ‘anha) that the Prophet (sallallahu ‘alayhi wa sallam) dispatched a man over a raiding party. The man would to lead the prayer and would always conclude his recitation with Soorah Ikhlaas. So upon their return they mentioned that occurrence to the Prophet (sallallahu ‘alayhi wa sallam) and He said to them,
“ask him why does he do that” so when they asked him he replied, “because it (i.e., the Soorah) a description of Ar-Rahmaan and I love to recite it.” So the Prophet (sallallahu ‘alayhi wa sallam) said, “inform him that Allah loves him.”