The 66th prohibition is that we are forbidden from leaving a person hung on a tree [after being executed by the High Court1] overnight. This is to prevent a person who would see him from repeating in his thought a curse against G‑d, since the only ones who are hanged are those who cursed G‑d or who worshipped idols. The verse also terms the latter as having "cursed G‑d."2

The source of this prohibition is G‑d's statement,3 "You may not allow his body to remain on the gallows overnight."

The Sifri4 says, "The phrase, 'You may not allow his body to remain on the gallows overnight,' is a prohibition.

The details of this mitzvah are explained in the 6th chapter of tractate Sanhedrin.5