Take it from a Canadian...
NO.
There is no scientific link between THC and muscle building. Nor is there any link between THC and any physiological impairment (other than the obvious one of inhaling smoke in your lungs). Nope. No credible science (Americans take note of the word "credible" here). There's lots of non-credible information that says the stuff will reduce your gains (and make your dick fall off and your wife cheat on you, etc) but that information was funded and/or published by extremly biased organizations to reduce drug use in the good 'ol US of A.
Just a whole lot of propaganda from the DEA, and ONDCP. Our senate committee up here in Canada did a two year investigation into Marijuana and it's effects and dangers. I am one of the few who actually bothered to read the entire report.
Keep smokin and don't worry bout your gains. Worry about your ass getting busted for a victimless crime, and getting treated like a murderer by your own government.