In the right place at the right time

 On   an October in  1954, 12 year old Cassius clay and one of his friend rode their bicycles to th Columbia
auditorium, for its annual  convention. The boys parked their bicycles and went around the bazaar for few  hours and decided to head back  home. But when they walked back to where they'd left their bikes, the new Schwinn bike of Cassius clay was gone.

Cassius  was in tears and went to a police officer standing near the basement of the building, where there was a boxing gym, the Columbia gym. Cassius was furious and demanded a statewide manhunt for his bike and threatened the beat hell out of the kid who had stolen it,

The police officer, named Joe Martin after listening to Cassius's oaths of revenge, said, "well do you know how to fight?"

Cassius said no, but I'd fight anyway. Martin said that perhaps the best thing to do would be to come around to the gym and learn how to fight before making any nasty challenges

Soon  Cassius   started coming to the gym on South florida street and learned the rudiments of boxing.
and  went on to became a legendary boxer
.
Which boxer am I talking about? Answer in Comments