Imam al-Shafi'i (may Allah have mercy on him) said: When Ibn al-Musayyib (may Allah have mercy on him) said that it is the Sunnah, it is most likely that it is from the Prophet (peace and blessings be upon him) or from the generality of his Companions. And it is not likely that Zayd (may Allah be pleased with him) said this from his own opinion, because opinion does not lead to it; and in what Sa'id (may Allah have mercy on him) called the Sunnah, since it goes against analogy and reason, there can be nothing but knowledge received through following (transmission), as we see it, and Allah knows best. We used to hold this view with that understanding, then I withheld from it, and I ask Allah for what is best. For we find some of them saying "it is the Sunnah", and then we do not find that his saying "it is the Sunnah" carries the force of being from the Prophet (peace and blessings be upon him); so analogy is more fitting for us in this matter. He said: And it is established from Zayd (may Allah be pleased with him) only in the same way as it is established from Sayyiduna Ali (may Allah be pleased with him).
It has been narrated from Sayyiduna Mu'adh bin Jabal (may Allah be pleased with him) from the Prophet (peace and blessings be upon him) with a chain the like of which is not established, and from Amr bin Shu'ayb, from his father, from his grandfather, from the Prophet (peace and blessings be upon him) with a weak chain, like the statement of Zayd bin Thabit (may Allah be pleased with him); and it is the view of the jurists of Madinah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الديات / 16312