Quotations about:
    shit

Everyone’s shit smells good to himself.

[Stercus cuique suum bene olet.]

Michel de Montaigne (1533-1592) French essayist

Essays, Book 3, ch. 8 (3.8), “Of the Art of Discussion [De l’art de conferer]” (1587) [tr. Screech (1987)]
    (Source)

Montaigne is recollecting an adage collected by Desiderius Erasmus in his Adagia (3.4.2, No. 2302). It's actually rendered there as Suus cuique crepitus bene olet. Erasmus maintains that the proverb is not meant literally, but metaphorically (that people value most things that are their own), though he does concede that people are more repulsed by others' excrement than their own.

Montaigne only presents the Latin, not a French translation (as is true with most of his Classical quotations). In context, he uses the phrase regarding how people criticize others for flaws that they, themselves, possess (and even consider virtuous, in their own cases).

I have also seen a version of this cited as an Icelandic proverb.

This essay (and passage) first appeared in the 2nd (1588) edition.

(Source (Latin)). Alternate translations:

Ev’ry mans ordure well,
To his owne sense doth smell.
[tr. Florio (1603)]

To each one their own manure smells good.
[tr. Cotton/Hazlitt (1877)]

Every man's filth smells sweet to himself.
[tr. Ives (1925)]

Each man likes best the smell of his own dung.
[tr. Zeitlin (1934)]

Every man likes the smell of his own dung.
[tr. Frame (1943)]

Every man's filth smells sweet to him.
[tr. Cohen (1958)]

Everyone thinks his own fart smells sweet.
[tr. Drysdall (2001); of Erasmus]