How did this all start? It apparently started with a track called "Number One" by Nelly, recorded for the Training Day soundtrack. Nelly's lyrics defended himself against his critics, stating "I'm tired of hearing rappers saying what's real hip-hop / but there album flops!" Many believed this was a direct diss to KRS-One, one of founding forefathers of Hip Hop, who had recently expressed his views on "real" and "fake" Hip Hop. 

On the song called, "Clear Em Out," KRS-One apparently replies to Nelly for allegedly dissing him.

Artist: KRS-One

Source: HHSO.com