Life brings no greater grief
Than happiness remembered in a time
Of sorrow.

[Nessun maggior dolore
Che ricordarsi del tempo felice
Ne la miseria.]

Dante Alighieri (1265-1321) Italian poet

The Divine Comedy [Divina Commedia], Book 1 “Inferno,” Canto 5, l. 121ff (5.121-123) [Francesca] (1309) [tr. James (2013), l. 141ff]
    (Source)

Francesca de Rimini is responding to Dante's request to speak of her love affair while in the middle of being punished for it. It is a true (if slanted) tale that occurred when Dante was a young man. Francesca da Polenta wed the crippled Giovanni Malatesta de Rimini, but fell in adulterous love with his brother, Paolo. Upon discovery of their affair, Giovanni killed them both. This was a local scandal, and would have been lost to time if Dante had not recorded it here. He relegates the lovers to the "least" eternal punishment in Hell, in the circle of carnal sins -- while Giovanni (who was still alive when this was written) is doomed to a lower circle for the murder (treachery to kindred). (More info.)

Inspiration for this particular phrase has been credited to many sources: Wisdom 11:11-12, Boethius (Consolation of Philosophy, 2.4.3-6), and Pindar (Pythian 4.510-512) are the most common. Augustine (Confessions 10.14) and Thomas Aquinas have also been cited.

(Source (Italian)). Alternate translations:

No greater grief assails us [...]
Than in unhappy hours to recollect
A better time.
[tr. Rogers (1782)]

Oh! how grievous to relate
Past joys, and tread again the paths of fate.
[tr. Boyd (1802), st. 23]

No greater grief than to remember days
Of joy, when mis'ry is at hand!
[tr. Cary (1814)]

No keener pang hath hell.
Than to recall, amid some deep distress,
Our happier time.
[tr. Dayman (1843)]

There is no greater pain than to recall a happy time in wretchedness.
[tr. Carlyle (1849)]

There is no greater grief
Than to remember happiness in woe.
[tr. Bannerman (1850)]

No greater grief than this,
Mem'ry to hold of the past happy time
In misery.
[tr. Johnston (1867)]

There is no greater sorrow
Than to be mindful of the happy time
In misery.
[tr. Longfellow (1867)]

No greater woe is there than to call to mind the happy time in your misery.
[tr. Butler (1885)]

There is no greater grief
Than to remember us of happy time
In misery.
[tr. Minchin (1885)]

There is no greater woe than in misery to remember the happy time.
[tr. Norton (1892)]

No deeper sorrow is, than to recall a time of happiness, in misery's hour.
[tr. Sullivan (1893)]

There is no greater sorrow
Than to recall to memory times of gladness
In misery.
[tr. Griffith (1908)]

There is no greater pain than to recall the happy time in misery.
[tr. Sinclair (1939)]

No grief surpasses this [...]
In the midst of misery to remember bliss.
[tr. Binyon (1943)]

The bitterest woe of woes
Is to remember in our wretchedness
Old happy times.
[tr. Sayers (1949)]

The double grief of a lost bliss
is to recall its happy hour in pain.
[tr. Ciardi (1954), ll. 118-19]

There is no greater sorrow than to recall, in wretchedness, the happy time.
[tr. Singleton (1970)]

There is no greater pain
than to remember, in our present grief,
past happiness!
[tr. Musa (1971)]

There is no greater sorrow
than thinking back upon a happy time
in misery.
[tr. Mandelbaum (1980)]

There is no greater sorrow
Than to think backwards to a happy time,
When one is miserable.
[tr. Sisson (1981)]

No sadness
Is greater than in misery to rehearse
Memories of joy.
[tr. Pinsky (1994), l. 107ff]

There is no greater pain than to remember the happy time in wretchedness.
[tr. Durling (1996)]

There is no greater pain, than to remember happy times in misery.
[tr. Kline (2002)]

There is no greater pain, I fear,
than to recall past joy in present hell.
[tr. Carson (2002)]

There is no sorrow greater
than, in times of misery, to hold at heart
the memory of happiness.
[tr. Kirkpatrick (2006)]

There is no greater sorrow
than to recall our time of joy
in wretchedness.
[tr. Hollander/Hollander (2007)]

No sadness afflicts the heart
More than recalling, in times of utter disaster,
Sweetened days in which we knew no darkness.
[tr. Raffel (2010)]

What's sadder than remembering
The happy past when you're feeling wretched?
[tr. Bang (2012)]