He completely devoted himself to fiqh. He was not concerned with tafsir of the Qur'an as was 'Ikrima, the client and student of Ibn 'Abbas and transmitter of his fiqh and tafsir. According to the tafsir of at-Tabari, "Yazid ibn Abi Yazid said: 'We used to ask Sa'id ibn al-Musayyab about the lawful and unlawful; he was the most knowledgeable of people. We asked him about the tafsir of an ayat of the Qur'an and he said, 'Do not ask me about any ayat of the Qur'an. Ask the one who claims that none of it is hidden from him,' meaning 'Ikrima." Sa'id met a great number of the Companions, and took from them and studied with them. What he especially sought were the judgements of the Messenger of Allah, may Allah bless him and grant him peace, and the judgements of Abu Bakr, 'Umar and 'Uthman. He took half of his knowledge from Zayd ibn Thabit, and most of his transmission was from Abu Hurayra, his father-in-law, since Sa'id was married to his daughter. He learned the fiqh of 'Umar from his companions to such an extent that he was considered the main transmitter of the fiqh of 'Umar. Ibn al-Qayyim called him "the transmitter of 'Umar and the bearer of his knowledge." Ja'far ibn Rabi'a said, "I asked 'Irak ibn Malik, 'Who among Malik's sources has the most fiqh?' He replied, 'The one among them with the most fiqh and knowledge of the judgements of the Messenger of Allah, may Allah bless him and grant him peace, the judgements of 'Umar, and the judgements of 'Uthman, and the one with the knowledge of what people did is Sa'id ibn al-Musayyab. The one with the most hadiths is 'Urwa ibn az-Zubayr. You could not wish for a greater ocean than 'Ubaydullah (ibn 'Abdullah ibn 'Utba),' 'Irak continued, 'I think that the one among them with the most fiqh is Ibn Shihab because he joined their knowledge to his.' Az-Zuhri said, 'I used to seek knowledge from three men: Sa'id ibn al-Musayyab, who had the most fiqh of all, 'Urwa ibn az-Zubayr, who was a bottomless ocean, and if you wish to find a kind of knowledge not found with anyone else you would find it with 'Ubaydullah.'" (I'lam, vol. 1, p. 18) Ibn al-Musayyab concentrated on fiqh. His concern with hadith was to learn the judgements of the Prophet, may Allah bless him and grant him peace, and he also learned the traditions containing the judgements of the khalifs since he was concerned to know the judgements and fatwas of the khalifs. The most prominent in his transmission of the knowledge of the fiqh of the Companions was 'Umar ibn al-Khattab, for his time was the pre-eminent time of fiqh, judgements and fatwas because the state was expanding and events occurred which made them necessary.
Since Ibn al-Musayyab followed the traditions of 'Umar in judgement and fiqh, ra'y (opinion) had great importance in his view because 'Umar frequently formed an opinion on matters about which there was no explicit text in the Book of Allah or the Sunna of the Messenger. So Ibn al-Musayyab also used ijtihad (independent reasoning) to answer problems presented to him about matters on which there was no explicit text from the Book or Sunna or judgement or fatwa of a Companion: he would give a fatwa based on his opinion which did not exceed what was proper. That is why it is transmitted that he used to give fatwa when others feared to do so.
He was the Imam of the fuqaha' of Madina in the time of the Tabi'un. He did not refuse to give a fatwa when there was need for one. His opinion was based on the firm pillars of fiqh: the Qur'an and hadith, and the judgements of the Prophet and Rightly-Guided Khalifs.
Imam Shafi‘i took as unquestionably authentic the ahadith that Sa‘id ibn al-Mussayyib narrated without mentioning the Companion from whom he received them. This means that, in the view of Imam Shafi‘i, Sa‘id ibn al-Musayyib was of the same rank as the Companions in knowledge and narration of the ahadith. Those who received Traditions from Sa‘id ibn al-Musayyib include Ata ibn Abi Rabah, Qatadah, Muhammad al-Baqir, a great grandson of Ali’s, az-Zuhri and Yahya ibn Sa‘id al-Ansari, among others.
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