MALVOLIO: In my
stars I am above thee, but be not afraid of greatness.
Some are born great, some achieve greatness, and
some have greatness thrust upon ’em.

William Shakespeare (1564–1616) English dramatist and poet

Twelfth Night, Act 2, Sc. 5, l. 147ff (2.5.147–150) (1601)
    (Source)

The phrase appears three times in the play:

1. As above, Malvolio reading the forged love letter he thinks is from Olivia.
2. Act 3, sc. 4, l. 42ff, Malvolio recalling the phrases from the letter.
3. Act 5, sc. 1, l. 393ff, the Fool reciting the second half of the phrase.

This was parodied in Joseph Heller, Catch-22, ch. 9 (1955), writing regarding Major Major: “Some men are born mediocre, some men achieve mediocrity, and some men have mediocrity thrust upon them.” See also Boorstin.