Tzvi B asks:
Where does it say that a mechalel shabbos goes to the seventh level of gehinom forever?
Tzvi B, Manchester
The Kollel replies:
Dear Tzvi,
Great to hear from you. Very excellent question!
I am not sure this is exactly what you were looking for, but it sounds very close, so I will share it: Zohar, Terumah 150b. See link below.
I hope this helps!
Warm regards,
Yishai Rasowsky
Link: