The Quran is transmitted in a set of accepted readings, and the fact is often misunderstood from outside as though it meant competing versions. It does not. The consonantal skeleton is one; the readings differ in vowelling, in the placing of a small number of consonantal points, in some morphological forms, in pausing, and in a few words where an alternative is preserved. Each reading is a transmitted performance of the same text, learned by ear from a teacher who learned it the same way, and its authority rests on that chain rather than on any manuscript.
Nafi was the recognised authority of Medina in the generation when these readings were being fixed, and he is reported to have learned from a number of teachers of the generation after the companions, taking what they agreed on. In the tenth century Ibn Mujahid set out seven readers as canonical, one for each of the principal centres, and Nafi is the Medinan among them; the list was later extended to ten. Each reader is known through named transmitters rather than directly, so a reading is always cited as the reading of one man through another, and the pairing is part of its identity.
The seven were not chosen arbitrarily. Ibn Mujahid selected readers whose recitation was already established in the great centres, Medina, Mecca, Kufa, Basra and Damascus, and whose chains of teachers could be traced, which made the canonisation a recognition of practice rather than an imposition on it. Readings outside the list continued to be studied and are recorded in the specialist literature; they are described as unusual rather than as wrong, and a few are cited in legal argument to this day.