For to a great man both things are needful; to treat trifles as trifles and important matters as important matters.

[Denn zu einem großen Manne gehört beides: Kleinigkeiten als Kleinigkeiten, und wichtige Dinge als wichtige Dinge zu behandeln.]

Gotthold Lessing (1729-1781) German playwright, philosopher, dramaturg, writer

Hamburgische Dramaturgie [Hamburg Dramaturgy], Essay 34, 1767-08-25 (1767-1769) [tr. Zimmern (1890)]
    (Source)

(Source (German)). Alternate translations:

For the great man does both, that is, he treats trivialities as trivialities and important things as important things.
[tr. Arons/Figal]

It is the mark of great people to treat trifles as trifles and important matters as important.
[Source]