Sefer HaChinuch ספר החינוך

That the witness not issue a ruling in the case that he is testifying about in capital punishments: That the witness not speak about the case that he is testifying about in capital punishments, except for his saying his testimony alone — and even though he is educated and wise; as the witness is not made into a judge in capital punishments, as it is stated (Numbers 35:30), “and a single witness should not respond about a soul for death.” And Rambam, may his memory be blessed, wrote (Sefer HaMitzvot, Negative Commandments 291), “And the negative commandment about this matter is repeated, as it is stated (Deuteronomy 17:6), ‘he shall not be killed by the mouth of one witness’ — meaning to say, he shall not be killed by the ruling of the witness. And they said in Sanhedrin 33b-34a, ‘“And a [...] witness should not respond about a soul,” whether for innocence or whether for guilt.’ And they explained that the reason for this is that it appears as if he is biased in his testimony. And this matter that he cannot respond — whether for innocence or whether for guilt — is only with capital punishments.”

Like that which is written in the commandment previous to this (Sefer HaChinukh 409) is from the roots of the commandment.

From the laws of the commandment is, for example, that which they, may their memory be blessed, said (Sanhedrin 34a) that a witness that testifies about capital punishments and afterwards says, “I have merit to teach about [the suspect],” is silenced; as we do not accept anything from him to rule upon [the suspect], since he is a witness. And to what do these words apply? To capital punishments; but with monetary law, the witness can teach for innocence or guilt, but he may not be counted among the judges (Sanhedrin 34b) and is not made a judge, since a witness is not made into a judge, even in monetary law. And to what do these words apply? To something that needs witnesses from Torah writ; but with something [rabbinic], a witness can become a judge. And because of this, he is made a judge in the validation of contracts; since it is established for us that the validation of contracts is rabbinic (Sanhedrin 18b). As from Torah writ, the signatories of the contract become as if their testimony was investigated in court. And the rest of its details are in Sanhedrin and in Makkot. (See Mishneh Torah, Laws of Testimony 5.)

And this commandment is practiced by males alone and at the time of the Temple with the Sanhedrin in their place, as then do we judge capital punishments, but not at another time. And it is then that we need the testimony of men. And a witness that testified and transgressed this and spoke, whether for innocence or whether for guilt, has violated this negative commandment. But he is not lashed for it, as there is no act [connected with it]. And Ramban, may his memory be blessed, explained this verse of “he shall not be killed by the mouth of one witness” (in the negative commandments that he added, commandment 9) [to be] about another negative commandment, and that is not to accept singular testimony in capital punishments. And that is, for example, that the witnesses saw [the suspect], one from this window, and [the other] from another window far from the first, to the point that those [two] standing at the window could not see one another, but they all saw the sinner. And this is elucidated in Tractate Makkot 6b.