Musar do Liczb 25:8

And [the law of a man] who has sexual relations with a maidservant is similar to those liable for the death penalties of the court in two ways. For our Rabbis, may their memory be blessed, said (Sanhedrin 81b), “One who has intercourse with an Egyptian woman, zealots may accost him,” like the matter which is written (Numbers 25:8). And this matter is more severe than the death penalties of the court, for they do not put to death, except with witnesses and a warning and by the decision of a Sanhedrin, which is not the case here.