Patience is bitter, but its fruit is sweet.

[La patience est amère, mais son fruit est doux.]

Jean-Jacques Rousseau (1712–1778) French philosopher and writer

Julie, or the New Heloise (Julie ou la Nouvelle Héloïse, Part 1, Letter 46 (from Julie) (1761) [tr. Stewart / Vaché]
    (Source)

Sometimes mis-cited to his Émile (1762). The sentiment is not necessarily original to Rousseau; the line is also credited to Aristotle, an ancient Chinese proverb, an ancient Persian proverb, etc.

The cited translation is from a newer source, but that way of wording it is extant by 1873. Another translation:

Patience is exceeding bitter; but its fruits are most delightfully sweet.
[tr. Kenrick (1784)]