“One thing I know, that I know nothing. This is the source of my wisdom.”

Source: Play It as It Lays

Variant: True knowledge exists in knowing that you know nothing.

Academy of Achievement interview (1991)
Context: I don't know who my parents were. I know nothing about my inheritance. I could be Jewish; I could be part Negro; I could be Irish; I could be Russian. I am spiritually a mix anyway, but I did have a solid childhood fortunately, because of some wonderful women who brought me up. I never had a father or a man in the house, and that was a loss, but you live with that loss.

Variant: The only thing that we know is that we know nothing — and that is the highest flight of human wisdom.
Source: War and Peace (1865–1867; 1869), Ch. I

One of the most commonly quoted forms.
The Serenity Prayer (c. 1942)
Variant: Lord, grant me the strength to accept the things I cannot change,
he courage to change the things I can,
and the wisdom to know the difference.

Widely known as The Serenity Prayer this has often been attributed to St. Francis, but earliest known forms of it appeared in the early 20th century, and it is generally credited to Reinhold Niebuhr.
Misattributed

Stanza 44.
Nosce Teipsum (1599)