I was only able to track 3 possible sources for this, there may very well be more. The earliest of the three was Daniel Gabriel Fahrenheit (1686-1736) in the prologue of his 1724 paper Experimenta et Observationes de Congelatione aquae in vacuo factae.
There are indications that this quote may have been stated more than a century earlier by the Danish astronomer Tycho Brahe (1546-1601), but I'm not able to locate anything specific.
Sorry I wasn't able to be of more help.
“Yesterday is gone. Tomorrow has not yet come. We have only today. Let us begin.” ~Mother Teresa (1910-1997)