Genius begins great works; but labour alone finishes them.

[Le génie commence les beaux ouvrages, mais le travail seul les achève.]

Joseph Joubert (1754-1824) French moralist, philosopher, essayist, poet

Pensées [Thoughts], ch. 23 “Des Qualités de l’Écrivain et des Compositions Littéraires [On Writers and Literature]” ¶ 52 (1850 ed.) [tr. Lyttelton (1899), ch. 22, ¶ 19]
    (Source)

(Source (French)). Alternate translations:

Genius begins beautiful works, but only labor finishes them.
[tr. Calvert (1866), ch. 8]

Genius begins great works; labour alone finishes them.
[tr. Attwell (1896), ¶ 335]

Beautiful works. Genius beings them, but labor alone finishes them.
[tr. Auster (1983)], 1801]