stitch in time, a

The proverb was originally a couplet. The number nine was apparently introduced fancifully for the sake of assonance.

A Stitch in Time May save nine.
[1732 T. Fuller Gnomologia no. 6291]

After a little while we acquired a method of keeping her [a boat] in the middle of the stream, by watching the moment she began to vary, and thereby verifying the vulgar proverb, ‘A stitch in time saves nine.’
[1797 F. Baily Journal 30 Apr. (1856) 268]

Repairing the ship. Found a crack or two in her inner skin. ‥A stitch in time saves nine.
[1868 Reade & Boucicault Foul Play I. ix.]

Looking after oneself is like looking after a house: a stitch in time‥
[1979 Homes & Gardens June 105]

Bibliography of major proverb collections and works cited from modern editions is available here.

A prompt action will avert more serious trouble. For example, Changing the car's oil every 7,000 miles is a stitch in time. The complete form of this adage, a stitch in time saves nine, appeared in Thomas Fuller's 1732 proverb collection, Gnomologia, and is so well known that it often is stated in shortened form. Ogden Nash played with it in the title for his verse collection, A Stitch Too Late Is My Fate (1938).