I do not love thee, Sabidius, nor can I say why;
I can only say this, I do not love thee.

[Non amo te, Sabidi, nec possum dicere quare:
Hoc tantum possum dicere, non amo te.]

Martial (AD c.39-c.103) Spanish Roman poet, satirist, epigrammatist [Marcus Valerius Martialis]

Epigrams [Epigrammata], Book 1, epigram 32 (1.32) [tr. Bohn’s (1859)]
    (Source)

(Source (Latin)). Alternate translations:

I love thee not, but why, I can't display.
I love thee not, is all that I can say.
[tr. Anon. (1695)]

I love you not, Sabidis, I cannot tell why.
This only can I tell, I love you not.
[tr. Amos (1858), 3.86, cited as 1.33]

I do not love you, Sabidius, nor can I say why;
I can only say this, I do not love you.
[tr. Bohn's Classical (1897)]

I do not love you, Sabidius; and I can't say why.
This only I can say: I do not love you.
[tr. Ker (1919)]

I love thee not, Sabidius; ask you why?
I do not love thee, let that satisfy!
[tr. Wright]

Mister Sabidius, you pain me.
I wonder (some) why that should be
And cannot tell -- a mystery.
You inexplicably pain me.
[tr. Wills (2007)]

I don’t love you, Sabidius, no, I can’t say why:
All I can say is this, that I don’t love you.
[tr. Kline (2016)]

There are two variations of this epigram of note. The first is from Thomas Forde (b. 1624):

I love thee not, Nell,
But why I can't tell;
Yet this I know well,
I love thee not, Nell.
[Letter to Thomas Fuller in Virtus Rediviva (1661)]

This seemingly served as a prototype for a more famous variant, attributed to Thomas Brown (1663-1704) (sometimes ascribed to "an Oxford wit") on Dr. John Fell, the Dean of Christ Church, Oxford, c. 1670:

I do not like thee, Dr. Fell,
The reason why I cannot tell;
But this, I'm sure, I know full well,
I do not like thee, Dr. Fell.
[Works, Vol. 4 (1774)]

This is sometimes rendered:

I do not love you, Dr. Fell,
But why I cannot tell;
But this I know full well,
I do not love you, Dr. Fell.