Hazrat Musleh-e-Maud (ra) states in one instance that the Holy Prophet (sa) appointed a group of teachers who taught the Holy Quran; they learned and memorised the entire Holy Quran from the Holy Prophet (sa) and taught it to others. There were four esteemed teachers who were responsible for learning the Holy Quran from the Holy Prophet (sa) and teaching it to others, and there were many companions under them who would also teach the Holy Quran to others. The names of these four esteemed teachers are Abdullah bin Mas‘ud (ra), Salim, the freed salve of Abu Hudaifah (ra), Muaz bin Jabal (ra) and Ubayy (ra) bin Kaab. The first two were Muhajireen and the latter two were Ansar. With regard to their work, Abdullah bin Mas‘ud (ra) was a labourer, Salim (ra) was a freed slave, while Muaz bin Jabal (ra) and Ubayy (ra) bin Kaab were chieftains of Medina. Thus, keeping in view every member of society, the Holy Prophet (sa) appointed qaris from each rank.
It is narrated in a hadith that the Holy Prophet (sa) used to say:
خُذُوا الْقُرْاٰنَ مِنْ اَرْبَعَةٍ (مِنْ) عَبْدِ اللّٰهِ بْنِ مَسْعُوْدٍ وَ سَالِمٍ وَ مُعَاذِ بْنِ جَبَلٍ وَ اُبَيِّ بِنْ كَعْبٍ
“Anyone who desires to read the Quran should learn from the following four individuals: Abdullah bin Mas‘ud, Salim, Muaz bin Jabal and Ubayy bin Kaab.”
These were the four people who either learned the entire Quran from the Holy Prophet (sa) or would recite it before him to ensure they did not make any mistakes. Aside from them, there were also many other companions who learned various portions of the Holy Qur’an directly from the Holy Prophet (sa).
(Dibachah Tafsir al-Quran, Anwar al-Ulum, Vol. 20, pp. 427-428)