Robert DeNiro fumes when he started an argument with Jay-Z at Leonardo DiCaprio's birthday party in New York.
The legendary actor allegedly scolded the rapper for failing to return not just one, but SIX of his phone calls.
The 69-year-old was sitting at a table at the upscale bash when the musician came over to say hello, but the Raging Bull star was in no mood to be friendly.
A New York Post insider then overheard De Niro complaining the rapper, who lives near the Oscar winner in the posh TriBeCa area, agreed to record a song for the Tribeca Film Festival, but failed to follow through.
The Mission favourite rang the music mogul on several occasions to plan the project, but his calls were never so much as returned.
The insider said: '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.'
The rapper tried make light of the situation by claiming he is terrible on the phone, but that failed to douse Robert's anger, and he continued to humiliate his foe in front of the likes of Martin Scorsese, Jamie Foxx and Cameron Diaz.
Even the feminine charms of his curvy wife Beyonce were not enough to get him off his high horse.
The source said: 'De Niro kept telling him that he thinks he’s the man, but that he was disrespectful.
'Beyoncé came over, but that didn’t calm Bob down.
'It was the talk of the party. 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.'