Love is but one thing with the gentle heart,
As in the saying of the sage we find;
Thus one from other cannot be apart,
More than the reason from the reasoning mind.

[Amore e ’l cor gentil sono una cosa,
Si come il saggio in suo dittare pone,
E cosi esser I’un sanza altro osa
Com’alma razional sanza ragione.]

Dante Alighieri (1265-1321) Italian poet

La Vita Nuova [Vita Nova; New Life], ch. 20, Sonnet 8, ll. 1-4 (c. 1294, pub. 1576) [tr. Norton (1867)]
    (Source)

The wise man referenced is the poet Guido Guinizzelli (or Guinicelli).

(Source (Italian)). Alternate translations:

Love and the gentle heart are but one thing,
As says the wise man in his apothegm;
And one can by itself no more exist
Than reason can without the reasoning soul.
[tr. Lyell (1845)]

Love and the gentle heart are one same thing,
Even as the wise man in his ditty saith:
Each, of itself, would be such life in death
As rational soul bereft of reasoning.
[tr. Rossetti (c. 1847; 1899 ed.)]

They are the same, Love and the gentle heart!
So runs the saw, which from the sage I stole;
Nor can they more abide, from each apart,
Than reason parted from the reasoning soul.
[tr. Martin (1862)]

Love and the noble heart are but one thing,
Even as the wise man tells us in his rhyme,
The one without the other venturing
As well as reason from a reasoning mind.
[tr. Reynolds (1969)]

Love and the gracious heart are but one thing,
As Guinizelli tells us in his rhyme;
As much can one without the other be
As without reason can the reasoning mind.
[tr. Musa (1971)]

Love and the gracious heart are a single thing, as Guinizelli tells us in his poem: one can no more be without the other than can the reasoning mind without its reason.
[tr. Hollander (1997), sec. 3]

Love and the gentle heart are one thing,
as the wise man puts it in his verse,
and each without the other would be dust,
as a rational soul would be without its reason.
[tr. Kline (2002)]

Love and the noble heart are one and the same thing,
as the sage states in his poem,
and one of them dares as little to exist without the other
as does the rational soul without reasoning.
[tr. Appelbaum (2006)]

Love and the open heart are always one,
the sage has written; neither love nor heart
can be until the other is begun,
as thought confirms a thinking counterpart.
[tr. Frisardi (2012), ch. 11]