Jay Z's former drug dealing partner says the pair managed to accumulate $100,000 by the age of 17.
De-Haven Irby, 44, and the rap star - real name Shawn Carter - grew up in Brooklyn, New York City, in the tough Marcy Houses housing project and his one-time best friend - who was also his neighbour - made money through a number of drug deals, in particular selling crack cocaine.
De-Haven told The Sun newspaper: ''I got into selling drugs at 14. Jay wasn't getting on with school and he used to come at weekends.
''Then he moved in with me and my aunt and uncle and I told him, 'What's mine is yours'. We made a lot of money. We were 17 and I'd have $100,000 (£62,700) stuffed under the mattress.''
The pair turned to crime in a bid to get rich quick and to escape their impoverished surroundings.
De-Haven explained: ''Thankfully, I never had to kill anyone. Although now I realise we were killing people slowly with the drugs. We just wanted money, clothes, jewellery, cars, a way up.''
De-Haven alleges his mother treated Jay - whose father abandoned his family when he was age 11, leaving him with his mother Gloria Carter and three siblings - like her own son but after the hip-hop star was catapulted into the spotlight following a string of hit records he forgot about the people in his past.
He added: ''Does my mum not deserve a call on Mother's Day? This is the lady who I woke up at four in the morning saying, 'Mum, Jay's in trouble, we have got to get him out of jail.' He got caught with drugs in his pocket.
''When she went to go and get him she didn't say, 'That's my neighbour,' she said, 'I've come to get my son.' ''
Jay Z is now believed to be worth a combined $1 billion with his wife Beyonce - with whom he has a 22-month-old daughter Blue Ivy.