小洞不堵，大洞受苦 [－－－－－－－－－]

Jyutping siu2 dung6 bat1 dou2 , daai6 dung6 sau6 fu2
Pinyin xiǎo dòng bù dǔ , dà dòng shòu kǔ

Definitions (CC-CEDICT)
  1. A small hole not plugged will make you suffer a big hole (idiom); A stitch in time saves nine.