When one rejects salah, it is like rejecting Allah, the Lord and Creator and, choosing Satan as a protector and friend. The Qur'an says,
"Whoever forsakes Allah and takes Satan as a friend, has of a surety suffered a manifest loss." (An-Nisaa 4:119)
"O Children of Adam! Let not Satan seduce you as he seduced your parents out of the Garden, stripping them of their raiment, to expose their shame; for he and his tribe see you from a position where you cannot see them; We made the devils friends (only) to those without Faith." (Al-A'raaf 7:27)
"Some He has guided; others have deserved the loss of their way; surely they took Satan as friend and protector instead of Allah and think that they are on the right path." (Al-A'raaf 7:30)
The most severe punishment of neglecting salah is being thrown out of the fold of Islam, therefore, becoming a kafir and it is well-known that the abode of the kafiroon is in the Hellfire. The Prophet (sallallahu alaiyhi wa sallam) said, "Hurry up with your salah when it is cloudy (lest you miss the correct time), for, to discard salah is to become a kafir."