The Torah states: "And one who steals a man and sells him, and he is found in his hand, he shall be put to death" (Exodus 21:16). The Mekhilta asks what this verse adds, since kidnapping is already addressed elsewhere. (Deuteronomy 24:7) states: "If a man be found to have stolen a soul of his brothers of the children of Israel."
The Deuteronomy verse requires that the kidnapper be "found," meaning caught in the act by witnesses. But found doing what? The verse addresses the act of stealing. Witnesses must observe the abduction itself. This gives us the requirement for witnesses to the kidnapping.
But kidnapping in Jewish law is a capital offense only when the kidnapper also sells his victim. The act of selling is a separate element of the crime that must also be established through testimony. Where does the Torah require witnesses to the sale?
The answer comes from (Exodus 21:16): "And one who steals a man and sells him." This verse mentions both the stealing and the selling as components of a single crime. The Mekhilta reads it as establishing the evidentiary requirement for both acts. Just as the stealing must be witnessed (per the Deuteronomy verse), the selling must also be witnessed (per the Exodus verse).
The practical consequence is that a kidnapper cannot be executed unless witnesses testify to both the abduction and the subsequent sale. Witnessing only the kidnapping is insufficient. Witnessing only the sale is insufficient. Both elements must be independently established through proper testimony. This double evidentiary burden made prosecution of kidnapping cases extremely rigorous, reflecting the Torah's consistent pattern of surrounding the death penalty with procedural safeguards that prevent unjust execution.