驷

一言既出，驷马难追

yī yán jì chū , sì mǎ nán zhuī

lit. once said, a team of horses cannot unsay it (idiom); a promise must be kept

君子一言，驷马难追

jūn zǐ yī yán , sì mǎ nán zhuī

a nobleman's word is his bond (proverb)