With regard to Hazrat Ubayy (ra) bin Kaab, it is mentioned that Allah the Exalted commanded the Holy Prophet (sa) to recite the Quran to Ubayy (ra). Furthermore, the Holy Prophet (sa) said that Ubayy (ra) was the best qari [who recites the Quran according to the rules of recitation] among his people.
(Ibn Saad, Al-Tabaqat al-Kubra, Vol. 3 [Beirut, Lebanon: Dar al-Kutub al-‘Ilmiyyah, 2017] 378)
It is mentioned in regard to him that he had great knowledge of the Holy Quran due to this very reason. Other narrations about this will be mentioned later.
Hazrat Musleh-e-Maud (ra) states:
“Hazrat Ubayy (ra) bin Kaab was among those four individuals with regard to whom the Holy Prophet (sa) said that they are the qurra-e-ummat [reciters of the Holy Quran] of this ummah.” That is, if anyone wished to learn to read the Quran, they should learn from them.
(Tafsir-e-Kabir, Vol. 10, p. 84)