I was just reading something about this phrase recently but I'm not finding it.
Here is a
2004 USA Today article on its sudden popularity in sports. William Safire wrote a
column about it in March 2006 where the first use he can find is from 1949 though increasing dramatically in use over the last decade.
He also points out two earlier versions of the phrase from vastly different sources:
"What I have written, I have written" - John 19:22
"I yam what I yam and that's all that I yam" - Popeye