The Prophet (sallallähu alaihi wasallam) said, "Allah will bring forth the severest Hour, and then all the Believers, men and women, will prostrate themselves before Him, but there will remain those who used to prostrate in the world for showing off and for gaining good reputation.
Such people will try to prostrate (on the Day of Judgment) but their back swill be as stiff as if it is one bone (a single vertebra)."
[Bukhari Book:60 No:441]