Where does the Bible say say what you mean and mean what you say?

That is not from The Bible. Neither is 'god helps those who help themselves', or 'money is the root of all evil'. The closest thing that the Bible says is this: 12But above all things, my brethren, swear not, neither by heaven, neither by the earth, neither by any other oath: but let your yea be yea; and your nay, nay; lest ye fall into condemnation.

A colloquial explanation of this would be as in the question 'mean what you say.' People should not need to add oaths to what they say to prove their sincerity. Likely if people do this in ordinary speech they are the more likely to be suspect.
Well the bible does not say exactly that but "let your yes be yes and your no be no" Matt 5:37.