说一不二 say one, not two
Explanation
形容说话算数，说到做到，从不反悔。
Describes someone who keeps their word, always keeps their promises, and never goes back on their word.
Origin Story
从前，在一个小山村里住着一对老夫妇，他们以诚实和勤劳著称。老夫妇有一个儿子，名叫小明，他继承了父母的优良品质，从小就懂得诚信的重要性。有一天，村里要修建一座水库，需要大家捐款出力。小明家虽然贫困，但小明还是毫不犹豫地捐出了家中仅有的积蓄。他向村长保证，一定尽全力帮助修建水库。在随后的日子里，小明每天都早早起床，到工地上帮忙。他干活认真负责，从不偷懒耍滑。村民们都被他的诚实和努力所感动，纷纷称赞他是一个说一不二的好青年。水库建成后，村民们的生活得到了极大的改善，小明也因此在村里赢得了极高的声誉。从此以后，“说一不二”就成为了形容小明为人处世的一个代名词。
Once upon a time, in a small mountain village, there lived an elderly couple known for their honesty and diligence. The couple had a son named Xiaoming, who inherited his parents' fine qualities and understood the importance of integrity from a young age. One day, the village was to build a reservoir, requiring everyone to donate money and effort. Although Xiaoming's family was poor, Xiaoming still unhesitatingly donated his family's only savings. He assured the village head that he would do his best to help build the reservoir. In the days that followed, Xiaoming got up early every day and went to the construction site to help. He worked diligently and responsibly, never being lazy or shifty. The villagers were all moved by his honesty and hard work, and they praised him as a good young man who kept his word. After the reservoir was built, the villagers' lives were greatly improved, and Xiaoming also gained a high reputation in the village. From then on, "Shuo Yi Bu Er" became a synonym for Xiaoming's conduct.
Usage
用于形容人说话算数，说到做到。
Used to describe someone who keeps their word and always fulfills their promises.
Examples
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他说话一向很靠谱，真是个说一不二的人。
tā shuōhuà yīxiàng hěn kǎopu, zhēnshi ge shuō yī bù èr de rén
He's always reliable in what he says; he's a man of his word.
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这次合作，你放心，我答应你的事一定会做到，我向来说一不二。
zhè cì hézuò, nǐ fàngxīn, wǒ dāying nǐ de shì yīdìng huì zuòdào, wǒ xiàng lái shuō yī bù èr
For this cooperation, don't worry, I will do what I promised, I always keep my word.