Ne ad scribendum cito prosilias et levi ducaris insania. Multo tempore disce, quod doceas.
St. Jerome, Letter CXXV to Rusticus (p. 428 in the Loeb Classics edition).
Roughly translated by me:
Do not swiftly rush into writing so as to be led by trifling folly. Learn for many years what you are to teach.
