The sin of a person who tells a lie against the Prophet (saws)

Narrated Abu Huraira:

The Prophet (ﷺ) said, "Name yourselves with my name (use my name) but do not name yourselves with my Kunya name (i.e. Abul Qasim). And whoever sees me in a dream then surely he has seen me for Satan cannot impersonate me. And whoever tells a lie against me (intentionally), then (surely) let him occupy his seat in Hell-fire."