Origin of quotation about boxing

Is this is?

I don't beat my opponents, they beat themselves
- Mayweather, Jr.

Some quotes applies in most of the situation, like
“It always seems impossible until it is done.” ~ Nelson Mandela
"Well done is better than well said.” ~ Benjamin Franklin
“One finds limits by pushing them.” ~ Herbert Simon
"Think Hard - Fight Tough" RDX Sports

I don't know the origin but this quote is really nice:

I think my grandmother saw my potential first. When I was young, I told her, 'I think I should get a job.' She said, 'No, just keep boxing.'
Floyd Mayweather, Jr.