The Moving Finger writes;
and, having writ, Moves on:
nor all thy Piety nor Wit
Shall lure it back to cancel half a Line,
Nor all thy Tears wash out a Word of it.

–From the Rubáiyát of Omar Khayyám, verse 51,  as translated by Edward Fitzgerald.

Believe it or not, I first read this in P.G. Wodehouse’s Thank You, Jeeves, where it was quoted in its entirely (by Jeeves, obviously). I particularly like the imagery invoked to describe how one cannot change the past.