Familiarity breeds contempt, while rarity wins admiration.

[Parit enim conversatio contemptum; raritas conciliat admirationem.]

Apuleius (AD c. 124 - c. 170) Numidian Roman writer, philosopher, rhetorician [Lucius Apuleius Madaurensis]

On the God of Socrates [De Deo Socratis]
    (Source)

First recorded passage with this phrase. Discussing why the gods do not mingle with humanity. Alternate translations:

• "Familiarity produces contempt, but infrequency conciliates admiration."
  [tr. Taylor (1822)]
• "Familiarity breeds contempt, but privacy gains admiration." [
  Works of Apuleius (1853)]
• "Familiarity breeds contempt, but concealment excites interest."
  [National Review (1858-04)]

The first part of the phrase is also used as the modern moral the English translation of Aesop's "The Fox and the Lion." Applying this proverb to Aesop seems to have first happened in 1820; in classic Greek sources, the moral was more along the line that "acquaintance overcomes fear."