DORINE: Those who have the greatest cause for guilt and shame
Are quickest to besmirch a neighbor’s name.

[Ceux de qui la conduite offre le plus à rire
Sont toujours sur autrui les premiers à médire.]

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

Tartuffe, or the Hypocrite [Le Tartuffe, ou L’Imposteur], Act 1, sc. 1 (1669) [tr. Wilbur (1963)]
    (Source)

(Source (French)). Alternate translations:

People, whose own Conduct is the most ridiculous, are always readiest to detract from that of others.
[tr. Clitandre (1672)]

They whose own conduct is the most ridiculous are always the first to slander others.
[tr. Van Laun (1876)]

The people whose conduct offers the greatest hold to ridicule are always the first to speak unkindly of others.
[tr. Wall (1879)]

People whose conduct is the most ridiculous, are always the readiest to run down that of others.
[tr. Mathew (1890)]

Those who are the most ridiculous in their own conduct are always the first to libel others.
[tr. Waller (1903)]

Those whose own conduct's most ridiculous,
Are always quickest to speak ill of others.
[tr. Page (1909)]

Those whose behavior is ridiculous
Always are first to see the faults of others.
[tr. Bishop (1957)]

Those whose behavior is most asinine
Are always more than ready to malign.
[tr. Frame (1967)]

Since they are always talked about,
They're sniffing other scandal out.
[tr. Bolt (2002)]

It's always the most ridiculous people who are the first to slander others.
[tr. Steiner (2008)]

People with the most to hide are always the worst.
Always the first to criticize others.
[tr. Campbell (2013)]

Those whose conduct gives room for talk
Are always the first to attack their neighbors.
[Bartlett's]