The Mark of Prostration (On the Forehead)
Question :
Is there any proof indicating that the mark that develops on the forehead from prostration is a sign of righteousness?
Answer:
This is not a sign of righteousness; rather, light on one's face, calmness in one's chest, good manners, and the like are. As for the mark that results from prostrating, it even shows up on the face of 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.
Source:
Ibn 'Uthaimin
Fatawa Islamiyah, Vol. 8 Page 330