What is the ruling on praying behind someone who changes the letter "Sad" to "Sin" in Al-Fatihah?

Reciting "As-Sirat" in Al-Fatihah with the letter "seen" (س) is one of the seven widely transmitted readings (qira'at sab'iyyah mutawatirah). It is the reading of Ibn Kathir and Ya'qub. Prayer behind someone who recites with this reading is valid by the consensus of scholars, because any reading that is widely transmitted and authentically established from the Prophet, peace and blessings be upon him, is valid.