Bayhaqi 16440 — The basis of qasamah, and beginning with the oaths

Imam al-Shafi'i (may Allah have mercy on him) said: Someone said to me: Why do you not take the narration of Ibn Bujayd? Imam al-Shafi'i (may Allah have mercy on him) replied: We do not know whether Ibn Bujayd heard anything from the Prophet (peace and blessings be upon him) or not; if he did not hear, then it is mursal, and neither we nor you regard the mursal as established. Sahl (may Allah be pleased with him) is a Companion and heard from the Prophet (peace and blessings be upon him), and he related the hadith in a manner that resembles only that of the most reliable narrators, so we take his narration for the reason I have described. Then he asked: Why do you not take the narration of al-Zuhri? I said: Because it is mursal; and the slain man was an Ansari, and the Ansar, through their concern with it, are more likely to know about it than others, when each of them is trustworthy — and each of them, by the grace of Allah, is trustworthy in our view.
Al-Shaykh (al-Bayhaqi) (may Allah have mercy on him) said: It seems that by the hadith of Ibn Shihab al-Zuhri he meant the hadith that follows.