The prayer bump can take the form of a discolouration of the skin, caused by repeated chafing and the build-up of callus. In extreme cases, the callus can be thick enough to create a real bump that portrudes from the forehead. Prayer bumps are common in certain Islamic countries, notably Egypt where they are colloquially called a 'zabiba' (raisin).
Prayer bumps are almost never present in women, rarely in establishment clerics, and are far more common in Egypt than in other countries; for these reasons they are generally seen as an intentional shibboleth of religious identity more than a passive product of regular prayer.
The Prayer Bump is referred to as a mark or trace of prostration in Koran 048:029