一yī诺nuò千qiān金jīn

Literal: one promise, a thousand pieces of gold
Figurative: A promise worth its weight in gold; said of someone whose word is utterly reliable and who always keeps a commitment.

Where does 一诺千金 come from?

Ji Bu was a man of Chu who served as a general under Xiang Yu, renowned for always honoring his word: people said a single promise from him was worth more than a hundred jin of gold. After Liu Bang defeated Xiang Yu and founded the Han dynasty, he put a price on Ji Bu's head, yet many sheltered and protected him because of his reputation, and in the end he was pardoned and served as a Han official. From his renown came the saying that "getting a hundred jin of gold is not as good as a promise from Ji Bu," later condensed into this idiom. The story is recorded by Sima Qian in the Records of the Grand Historian.

Source (出处): 《史记·季布栾布列传》

How is 一诺千金 used? Two examples

他tā答dā应yìng的de事shì一yī定dìng会huì做zuò到dào，真zhēn是shì一yī诺nuò千qiān金jīn。
Whatever he promises, he always carries out; his word really is worth its weight in gold.
做zuò生shēng意yì要yào讲jiǎng信xìn用yòng，答dā应yìng了le就jiù得děi一yī诺nuò千qiān金jīn。
In business you have to be trustworthy; once you've promised something, your word must be as good as gold.

In Wanyu, all 400 chengyu have audio on every sentence, and you can save them to your decks and review them with spaced repetition. Free.