8:00 AM PT -- Doctors say Jim Kelly's surgery was a success.

He is resting comfortably at a hospital in New York ... and is expected to stay there for several weeks.

With Jim Kelly going into surgery today for his recurring oral cancer, fellow Bills legend Andre Reed has recorded a message of support for his former QB -- and TMZ Sports has the exclusive audio.

In the emotional letter Andre penned and narrated, Reed calls Kelly "one of the strongest men I've ever met"... and tells his longtime Buffalo teammate he knows he can beat the disease -- something Jim's already done twice in his life.

"You've quarterbacked through a lot of things in the last 20 years of your life. I know you will take this challenge head on without backing down like you always do."

"I and my family are thinking of you, along with Bills fans and the NFL family all across the country."

"12 + 83 will always equal 6. I love your fight. I love you, 12."

FYI, Kelly sent a thank-you message out to all of his fans the day before going under the knife ... and it's gonna make you root for him that much harder.

Get well, Jim!!