Never leave that till tomorrow which you can do today.

Benjamin Franklin (1706-1790) American statesman, scientist, philosopher, aphorist

“The Way to Wealth” (1758)
    (Source)

Today, this is more commonly given as "Never put off till tomorrow what you can do today."

Franklin had used a different phrasing in Poor Richard (1742 ed.): "Have you somewhat to do To-morrow, do it To-day." That was reprinted in Poor Richard Improved (1758 ed.), but when that latter work was condensed into "The Way to Wealth" that same year, the wording above was used.

As with so many of Franklin's "Poor Richard" aphorisms, this was not original to him. Thomas Fuller uses a similar phrase in 1725.

The sentiment itself has been mocked or modified by others such as Mark Twain, Josh Bilings, Aldous Huxley, Pablo Picasso, and Mignon McLaughlin.