一言既出，驷马难追

idiom

once a word is spoken, it cannot be taken back even by a team of four horses; what's said cannot be unsaid; a promise must be kept

Used to emphasize the importance of keeping one's word or the irreversibility of spoken words. Often used as a warning to be careful about what one says, or as a reminder that a promise is binding. This idiom is commonly quoted to stress the value of integrity and the finality of verbal commitments.