If I had eight hours to chop down a tree, I’d spend six sharpening my axe.

Abraham Lincoln (1809–1865) American lawyer, politician, US President (1861–65)

(Spurious)

Commonly attributed to Lincoln, but there is no written proof he ever said it. The phrase can only be traced back to the 1940s, and is first credited to Lincoln in the 1960s; it's often attributed to an anonymous lumberjack / woodcutter.

Variants include different numbers of hours, or couching it in minutes, or rephrasings such as:

Give me eight hours to chop down a tree, and I will spend the first six sharpening the axe.
 

For more discussion about and research into this quote, see Quote Origin: To Cut Down a Tree in Five Minutes Spend Three Minutes Sharpening Your Axe – Quote Investigator®.