In explaining this term, many cite the following hadith—even though its chain is weak: "Sometimes heedlessness and distraction come over my qalb (the heart). I seek forgiveness and perform tawba (repentance) one hundred times a day." (Bukhari, Daʿawāt, 3; Tirmidhi, Daʿawāt, 38; Musnad, I, 388) They say: Gayn is a ḥāl (spiritual state) that would befall the Prophet's qalb, and he would perform tawba from it. This state is likened to a mirror that loses its luster when someone breathes upon it, but once the breath dissipates, the mirror's brightness reappears. (Lumaʿ, 451)
Junayd said: Reyn is from among waṭanāt (maqām or habitual states); whereas gayn is from among khaṭarāt (passing thoughts). Waṭan is lasting, while khaṭir is transient. Its example is as follows: No matter how many expert polishers gather, they cannot make a mirror from a stone. But when a mirror rusts, it can only be cleaned with a tool. For rust and blackness are essential elements in stone, while light is an essential element in a mirror. The essence remains; these transient qualities are not permanent. (Hujwīrī, 8)
Gayn is a veil over the qalb. This veil is removed by seeking maghfira (forgiveness). 288 ghayr
This (veil) is of two kinds: one is transparent, the other thick. The thick one is found in those who are heedless or commit major sins. The transparent one is present in every creature, whether prophet or walī (friend of God). (Hujwīrī, 474)
Gayn is different from reyn. Gayn is rust. This rust is a subtle veil that prevents purification. This veil disappears with the light of manifestation along with the persistence of iman (faith). Reyn, on the other hand, is a thick veil that stands between the qalb and iman, obstructing the Real (God). Gayn is distraction from being shāhid (witness) with correct belief, and veiling from it. (Kāshānī, 168; Jāmiʿ, 91)