1019 – Ibn ʿUmar, may Allah be pleased with them both, relates that the Prophet ﷺ said, “When one man restrains another and a second man kills him, the killer is to be put to death, and the one who held him is to be imprisoned.” Al-Dāraquṭnī transmitted the report both with a continuous chain and as mursal (with a missing link). Ibn al-Qaṭṭān declared it sound, and its transmitters are trustworthy, yet al-Bayhaqī considered the mursal version weightier. (1)

Hadith grading:
The report is mursal.
The compiler wrote: “Al-Dāraquṭnī recorded it both with and without a continuous chain; Ibn al-Qaṭṭān authenticated it, and its narrators are reliable, but al-Bayhaqī preferred the mursal form.”
Al-Ḥāfiẓ Ibn Kathīr said in al-Irshād: “Its chain fulfils the conditions set by Muslim.”
Al-Shawkānī observed: “Al-Dāraquṭnī said the mursal version is more common, and al-Bayhaqī judged the mursal to be sounder; the connected chain is therefore not preserved.”

Points derived from the hadith:
1 – Our Ḥanbalī jurists state: If one person restrains someone so that a third may kill him, and the third does kill him, the killer is executed—there is unanimous agreement on this—because he deliberately and unjustly killed his equal. As for the one who restrained the victim, he is imprisoned until he dies; no retaliation (qiṣāṣ) is taken against him and no blood money is owed.
2 – This ruling applies only if the restrainer knows that the killer intends to take the victim’s life. If he is unaware of that—such as when they were engaged in horseplay or jest—nothing is due from the restrainer, for the death was not caused by his action, and an intention to kill is not ascribed to him.

This is the well-known opinion in the school of Aḥmad and is one of its distinctive positions.