I first saw that quote a while back with a credit to -Buddah for saying it, but ultimately I learned that you can’t believe everything you read on the internet. That doesn’t make it any less powerful though.
Some digging revealed a more likely source of that quote.
There is one simple thing wrong with you – you think you have plenty of time … If you don’t think your life is going to last forever, what are you waiting for? Why the hesitation to change? You don’t have time for this display, you fool. This, whatever you’re doing now, may be your last act on earth. It may very well be your last battle. There is no power which could guarantee that you are going to live one more minute.
Journey to Ixtlan: The Lessons of Don Juan by Carlos Castaneda