If you do not expect the unexpected, you will not find it; for it is hard to be sought out and difficult.
 
[ἐὰν μὴ ἔλπηται ἀνέλπιστον οὐκ ἐξευρήσει, ἀνεξερεύνητον ἐὸν καὶ ἄπορον]

Heraclitus of Ephesus (c.540-c.480 BC) Greek philosopher [Ἡράκλειτος, Herákleitos, Heracleitus]

Fragment 18 [tr. Burnet (1920), DK B18]
    (Source)

(Source (Greek)). Alternate translations:

He who does not expect will not find out the unexpected, for it is trackless and unexplored.
[tr. Kahn (1981), VI (D. 18)]

He who does not expect the unexpected will not find it out.
[tr. Kahn (1981), VI (D. 18), variant]

He who does not expect the unexpected will not find it, since it is trackless and unexplored.
[tr. Allan (2008)]

Unless you expect the unexpected, you will not find it, for it is hidden and thickly tangled.
[tr. Jenks (2014)]