The first person that I heard espouse "do what you love" was Chris Wanstrath of GitHub fame. He covered it in his Startup Riot keynote a few years back.

The guy that said it best was Steve Jobs in his 2005 Stanford commencement address.

I am saddened by his resignation as the CEO of Apple. But strengthened by his words. 

Love what you do. Know that you are going to die. Stay hungry. Stay foolish.