The Mark of Prostration (On the Forehead)
Is there any proof that the mark that develops on the forehead from prostration is a sign of righteousness?
This is not a sign of righteousness; rather, light on one’s face, calmness in ones chest, good manners, and the like are. As for the mark that results from prostrating, it even shows up on the face on those people who only pray the obligatory prayers, from some kind of sensitivity in their skin. And it does not appear on the faces of some people who pray often and who prolong their prostrations.
Shaykh Muhammad bin Saalih al-`Uthaymeen
Fatawa Islamiyah vol.8, p.330 DARUSSALAM