'Heard melodies are sweet, but those unheard are sweeter' - who quoted this line?

- John Keats English Romantic lyric poet who devoted his short life to the perfection of a poetry marked by vivid imagery, great sensuous appeal, and an attempt to express a philosophy through classical legend.

Ode on a Grecian Urn
BY JOHN KEATS
Heard melodies are sweet, but those unheard
Are sweeter; therefore, ye soft pipes, play on;
Not to the sensual ear, but, more endear'd,
Pipe to the spirit ditties of no tone:
Fair youth, beneath the trees, thou canst not leave
Thy song, nor ever can those trees be bare;
Bold Lover, never, never canst thou kiss,
Though winning near the goal yet, do not grieve;
She cannot fade, though thou hast not thy bliss,
For ever wilt thou love, and she be fair!

[Source: Britannica.com and Poetryfoundation]