John Lightfoot on John 8:5

[That such should be stoned.] Such. Who? what, all adulteresses? or all taken in adultery, in the very act? There is a third qualification still: for the condition of the adulteress is to be considered, whether she was a married woman, or betrothed only.

God punisheth adultery by death, Leviticus 20:10. But the masters of traditions say, that "wherever death is simply mentioned in the law," [that is, where the kind of death is not expressly prescribed,] "there it is to be supposed no other than strangling." Only they except; "a daughter of an Israelite, if she commit adultery after she is married, must be strangled; if only betrothed, she must be stoned. A priest's daughter, if she commit adultery when married, must be stoned; if only betrothed, she must be burnt."

Hence we may conjecture what the condition of this adulteress was: either she was an Israelitess not yet married, but betrothed only; or else she was a priest's daughter, married: rather the former, because they say, "Moses in the law hath commanded us that such should be stoned." See Deuteronomy 22:21. But as to the latter, there is no such command given by Moses.