Al-Layth Ibn Saʿd Ibn ʿAbd al-Raḥmān al-Fahmī al-Qalqashandī (الليث بن سعد بن عبد الرحمن الفهمي القلقشندي) was the chief representative, Imam, and eponym of the Laythi school of Islamic jurisprudence from Egypt and of Persian origin.
There is agreement on his Persian origin.
Muhammad Ibn Ahmad Ibn ‘Othman AL-DHAHABI (Al-Turkistani) says in his Siyar A’lam Al-Nubala`:
ابن عبد الرحمن ، الإمام الحافظ شيخ الإسلام ، وعالم الديار [ ص: 137 ] المصرية أبو الحارث الفهمي مولى خالد بن ثابت بن ظاعن . وأهل بيته يقولون : نحن من الفرس ، من أهل أصبهان . ولا منافاة بين القولين .
“Ibn Abdul-Rahman, the Imam, the Hafidh, the Shaykh Al-Islam, the scholar of the lands of Egypt, Abu Al-Harith Al-Fahmi MAWLA (client) of Khaled Ibn Thabit Ibn Dha’en and his family used to say: ‘We are PERSIANS from ISFAHAN’ and there is no contradiction in what they said (as Isfahan is one of the ancient Persian cities).”
It can easily be argued that the vast majority of the non-Arab Salaf were of Persian origin, in particular the major scholars of ALL fields (Fiqh, Hadith, Arabic Grammar etc. etc.), especially in the lands of Iraq (that were part of the Persian empire). The service of Persian Sunni scholars to Islam is only understood by those who have studied the early history of Islam and are familiar with the Ansab (lineage) of the scholars and the Sunni history of Persia.