Imam al-Qāsim ar-Rassi, upon him be peace, was based in Medina around the 2nd century after the Hijra. He was a theologian, philosopher, jurist, scholar, and imam who spent the majority of his time in the City of his Grandfather, the Prophet Muhammad, peace and blessings be upon him and his progeny. Although he is more famous for his knowledge of theology and dialectics, he nevertheless was referred to regarding his jurisprudential opinions as well.
Most of his jurisprudence can be derived from his replies to the issues and questions of his time. Some of these works include: Masā`il Ja’far bin Muhammad an-Nayrūsi, Masā`il ‘Abdullah bin Muhammad al-Kallāri, Kitāb at-Tahāra, Kitāb as-Salāt al-Yawm al-Layla, Masā`il ‘Ali bin Jahshayār, Kitāb al-Farā`iđ wa as-Sunan, and Kitāb al-Manāsik. His jurisprudential positions were not written by him in a book rather, they were compiled in various fatwas and aforementioned replies to questions by his students.
Those unfamiliar with the development of the Zaydi School remark that since Imam al-Qāsim, upon him be peace, barely referenced Imam Zayd, upon him be peace, in his works, the former can’t be considered a Zaydi authority. We reply by saying that although Imam ar-Rassi hardly referenced Imam Zayd, upon him be peace, in his jurisprudential positions, he is nonetheless “Zaydi” in the sense that he relied upon the collective opinion of the Ahl al-Bayt in his rulings. This is the hallmark feature of the Zaydi School which differentiates it from any other School. For example, when he was asked about the permissibility of wiping over the leather socks (khuffayn), he said:
As for wiping over the leather socks, the Ahl al-Bayt concur
That it is not permissible and invalid if that were to occur.
Therefore, Imam ar-Rassi, upon him be peace, is considered a Zaydi authority despite the fact that he barely referenced Imam Zayd bin ‘Ali specifically, upon them be peace.
It is also noteworthy that when Imam al-Qāsim, upon him be peace, made the call of the Imamate, among those that took the oath of allegiance to him was the grandson of Imam Zayd, Ahmed bin ‘Isa, upon them be peace. Imam Ahmed bin ‘Isa, upon them be peace, inherited the legacy of his grandfather in that he was recognised as “the Jurist of the Messenger’s Progeny (Faqīh Āli ar-Rasūl).” Yet, when he and other notables of the Prophet’s Progeny, like ‘Abdullah bin Mūsa and al-Hasan bin Yahya (another descendant of Imam Zayd) took the oath of allegiance to Imam al-Qāsim in 220 AH, they declared: “You have more right to this matter than we do by virtue of your knowledge!” Thus, two descendants and inheritors of Imam Zayd bin Ali, upon them be peace, acknowledged the qualifications of Imam al-Qāsim, upon him be peace, and followed him.
The jurisprudential book Al-Jāmi’ al-Kāfi fī Fiqh az-Zaydiyya lists the rulings of the four pillars of Zaydi jurisprudence and hadīth on various legal matters: Imam al-Qāsim bin Ibrāhīm, Imam Ahmed bin ‘Isa, Imam al-Hasan bin Yahya, and Muhammad bin Manšūr al-Murādi, upon all of them be peace. Suffice to say that not only was Imam ar-Rassi, upon him be peace, considered a Zaydi jurist, he was also considered an authority.