It is not what we do, but also what we do not do, for which we are accountable.

[Ce n’est pas seulement pour ce que nous faisons que nous sommes tenus responsables, mais aussi pour ce que nous ne faisons pas.]

Molière (1622-1673) French playwright, actor [stage name for Jean-Baptiste Poquelin]

(Attributed)
    (Source)

The French phrase comes up in search, but, again, without citation beyond "Molière".

The earliest reference I can find to this phrase (in English) is in Maturin Ballou's Treasury of Thought (1871), without citation to where Molière might have said it. It is likely a paraphrase, but I can find no source material that recalls it. A more recent (but no better documented) variant:

It is not only for what we do that we are held responsible, but also for what we do not do.
 

See also Marcus Aurelius (c. AD 170), Fuller (1725), Mill (1859), Sewell (1877), Harris (1953), Nash (1959) (also), Masson (1988).