He that avoideth not small faults, by little and little falleth into greater.

[Qui parvos non devitat defectus, paulatim labitur ad majora.]

Thomas von Kempen
Thomas à Kempis (c. 1380-1471) German-Dutch priest, author
The Imitation of Christ [De Imitatione Christi], Book 1, ch. 25, v. 10 (1.25.10) (c. 1418-27) [tr. Anon. (1901)]
    (Source)

Cross-referenced in some sources to Ecclesiasticus (Sirach) 19:1 -- "One who despises small things will fail little by little."

(Source (Latin)). Alternate translations:

He that will not flee small sins, shall by little and little fall into greater.
[tr. Whitford/Raynal (1530/1871)]

He who will not flee small sins will, by little and little, fall into greater sins.
[tr. Whitford/Gardner (1530/1955)]

He that avoideth not small slips, by litle and litle may take a great fall.
[tr. Page (1639), 1.25.39]

He who does not inure himself to vanquishing by subduing less temptations, will never be able to grapple with more violent and trying ones; and infirmities once yielded to, grow insensibly to stubborn habits of vice.
[tr. Stanhope (1696; 1809 ed.), "The Christian's Pattern"]

He who is not careful to resist and subdue small sins, will insensibly fall into greater.
[tr. Payne (1803), 1.25.12]

He that avoideth not small faults, by little and little falleth into greater.
[ed. Parker (1841)]

He who is not careful to resist small sins, will insensibly fall into greater.
[tr. Dibdin (1851)]

He that does not shun small defects, by little and little falls into greater.
[ed. Bagster (1860)]

He who shunneth not small faults falleth little by little into greater.
[tr. Benham (1874)]

He who does not overcome small faults, shall fall little by little into greater ones.
[tr. Croft/Bolton (1940)]

He who does not try to shun small faults slips little and little into greater ones.
[tr. Daplyn (1952)]

The man who doesn't keep clear of petty faults will gradually slip into graver ones.
[tr. Knox-Oakley (1959)]

The man who does not avoid small failings gradually drifts into greater ones.
[tr. Knott (1962)]

If you do not avoid small faults, you will soon commit greater ones.
[tr. Rooney (1979)]

The person who does not avoid small faults, little by little slips into greater ones.
[tr. Creasy (1989)]