(Devarim 17:6) "By word of two witnesses or three witnesses is the dead one to be put to death": This tells me only of those who are put to death in this context. Whence do I derive (the same for) the others who are put to death (by beth-din)? From "By word of two witnesses or three witnesses is the dead one to be put to death."
Whence is it derived that a (single) witness is not to testify for incrimination? From "He shall not be put to death by word of one witness."
And whence is it derived that a disciple shall not speak for incrimination? From "He shall not be put to death by word of one witness" … And the sages say: Whence is it derived that a (single) witness is not to testify for exoneration? From (Bamidbar 35:20) "and a witness shall not testify in a soul." Whence is it derived that a disciple shall not speak for incrimination? From "one … shall not testify in a soul to kill him."