Wù yǐ è xiǎo ér wéi zhī, suíshǒu luàn rēng lājī yě shì bù wénmíng xíngwéi.
Don't commit even small wrongs - casually dropping litter is uncivilized too.
Tips
history
From the deathbed edict of Shu Han founder Liu Bei 刘备 to his son Liu Shan, recorded in 《三国志·蜀书·先主传》裴松之注: 勿以恶小而为之,勿以善小而不为 - 'Do no evil, however small; miss no good, however small.' A textbook Confucian statement on moral consistency.
usage
Almost always paired with 勿以善小而不为. Standard quotation in civic-education and anti-corruption campaigns.