说一不二 [- yī bú èr]mean what one says; He is a man of his word.; He is as good as his word.; never make a promise one cannot keep; say one and it never becomes two — one never changes his words; stand by one's word:
•¶ 他是说一不二的， 答应的事一定能办到。 He is a man of his word, and never makes a promise he can't keep.