Will he who kills his wife and her lover upon seeing them commit adultery be killed for it, and is this what the Prophet cited as a precedent for a Companion?

If a man finds his wife committing adultery, he must bring four witnesses to prove the adultery before the Sharia court to establish the prescribed punishment (حد) on the wife and the adulterer. If he does not have witnesses, he must resort to mula'anah (mutual imprecation) before the court.

However, if he kills them, and there are four witnesses to the adultery, or the heirs of the deceased admit to it, then there is no qisas (retribution) or diyah (blood money) upon the killing husband. This is based on what was narrated from Ali and Umar, may Allah be pleased with them.

Some scholars are of the opinion that the husband is not held accountable for killing the adulterer if the adulterer was muhsan (married), because he deserves to be killed. Others believe that the ruling of no accountability applies to both the muhsan and non-muhsan equally if the adultery is proven by evidence or by the admission of the guardian of the blood.

Ibn al-Qayyim differentiated between what the husband may do between himself and Allah, which is the permissibility of killing, and what his word is accepted for outwardly, which is that his claim is not accepted except with evidence (four witnesses). He emphasized that the hadith of Sa'd ibn Ubadah does not mean the outward permissibility of killing the adulterer without evidence, but rather it is an affirmation of Sa'd's jealousy, while stressing that Allah and His Messenger are even more jealous. Allah has legislated evidence to preserve lives and to block the means to corruption.