This isn't how Tory Lanez wants to get out of prison, but he is -- after suffering a prison yard stabbing so bad, he needs hospital treatment ... TMZ has learned.

Tory was in the yard of California Correctional Institution at Tehachapi Monday morning when another inmate stabbed him ... according to multiple sources, including one in law enforcement.

It's unclear what the motivation was for the attack, but one source tells us Tory was rushed by ambulance to a nearby civilian hospital in Bakersfield.

Just last week, he posted a pic of himself looking pretty buff behind bars, and bragging his next album would be dropping soon.

Source: Fox11