Being good is better far than seeming so.

[Refert sis bonus, an velis videri.]

Martial (AD c.39-c.103) Spanish Roman poet, satirist, epigrammatist [Marcus Valerius Martialis]

Epigrams [Epigrammata], Book 8, epigram 38 (8.38.7) (AD 94) [tr. Francis & Tatum (1924), ep. 415]
    (Source)

(Source (Latin)). Alternate translations:

'Tis better bee, than seeme, good.
[16th C Manuscript]

'Tis not the same,
To covet and to merit a good name.
[tr. Hay (1755)]

Bounteous to be, or seem; the distance wide!
[tr. Elphinston (1782); Book 2, ep. 111]

It makes a difference whether a man is, or only wishes to seem, good.
[tr. Bohn's Classical (1859)]

It matters much whether thou'rt truly good, or would'st appear so.
[ed. Harbottle (1897)]

Wide is the difference 'twixt goodness and pretence.
[tr. Ker (1919)]

... the great gulf ’twixt goodness and pretence.
[tr. Pott & Wright (1921)]

There is a difference between goodness and pretence.
[tr. Shackleton Bailey (1993)]