Disco is so distinct, it should have an origin right? There isn't a truly first "disco" song. The distinction between early-mid 70s soul, funk and "disco" is difficult to discern. But I'd say the closest thing would be The Love I Lost, because of the drums and tempo. Others had used a 4 on the floor beat before but not as the backbone of the entire track the way Gamble and Huff did on that song. The baseline, strings and wah-wah guitars set a precedent that was readily imitated by other acts for the remainder of the decade.
In "Turn the Beat Around: The Secret History of Disco" by Peter Shapiro, the first two disco songs, in his opinion, came out in 1973:
The Temptations "Law Of The Land" Harold Melvin and the Blue Notes "The Love I Lost"
Of course, there was dance music before that but he was referring to the coalescence of disco as a musical style/genre.