The first rap song to reach the top 40 was "Rapper's Delight." It was not the first rap record released. That belongs to The Fatback Band's "King Tim III." Fatback beat Sugar Hill by a few months, but did not score a pop hit....possibly because Fatback's record label released the song as a B side. Plus, it wasn't a rip off of a current disco hit.
One could argue that rap appeared on vinyl long before 1979. A 1972-73 album by Lightenin' Rod, from the proto-rap group The Last Poets, released an all rhyming album called "Hustler's Convention." (With backing music by Kool and the Gang and other well known performers.) Not only is it all in rhyme, the content is identical to the gangsta rap albums that would come 20 years later.
Or we could look at the original rappers...guys and gals who were doing rap before rap meant rhyming. Isaac Hayes, Millie Jackson, Barry White, etc.
"Rapture" was the first #1 hit....to feature a rap. Many people don't realize how influential Blondie was in the underground hip hop scene.