Начатого дела не бросай

See Затянул песню, так веди до конца (3)

Var.: Начатое дело доводи до конца

Cf: Do nothing by halves (Am.). If a task is once begun, never leave it till it's done (Am.). Never do things by halves (Br.). Never give up once you have started (Br.). Never leave a task until it is done (Am.). Never say A without saying В (Am.)