In the world of hip-hop, Jay-Z rules, and in the world of mafia movies and comedies, Rober De Niro rules. But if these two worlds collide, it could be quite explosive. Music-new.com reports that Robert De Niro reportedly slammed Jay-Z for being "disrespectful" at Leonardo DiCaprio''s birthday party. The bash was held at New York City supper club The Darby on Saturday night, to celebrate Leonardo''s 38th birthday. A host of stars were in attendance, but it’s been claimed all eyes were on actor Robert and rapper Jay-Z. The Hollywood star apparently took the hip-hop mogul to task for not returning his phone calls. It''s been claimed Jay-Z had agreed to record a track for the Tribeca Film Festival, which Robert was involved with. The song didn’t materialise, although Robert tried desperately to reach out to Jay-Z. "Bob was sitting at a table, and when Jay-Z went over to say hello, De Niro told Jay that he never called him back," an insider told New York Post. "Bob wasn't in any mood to make polite conversation. He told Jay that if somebody calls you six times, you call them back. It doesn't matter who you are, that is just rude. Another source added: "De Niro kept telling him that he thinks he's the man, but that he was disrespectful.'" The rapper apparently told Robert that he isn’'t great on the phone, which is why the miscommunication occurred. Jay-Z''s wife Beyoncé Knowles was also at the celebration and reportedly tried to calm the mood. Although the guests eventually got back into the swing of the party, they reportedly couldn’t stop discussing the incident. "Everyone was saying there's only one star in New York bigger and badder than Jay-Z, and that's Robert De Niro. He can be quite scary when he's angry," one insider claimed. A source who is friends with the actor has insisted they merely had a "low-key conversation". Neither of the stars'' representatives have commented. Many other famous faces turned out to celebrate Leonardo’s birthday, including Cameron Diaz, Mark Ruffalo, Emma Watson, Chris Rock, Jamie Foxx and Jonah Hill.