Shaykh Muhammad al Banna رحمه الله said, “The position of Ash Shafi’ee رحمه الله was that If you catch the Imam while standing, then fatiha is obligatory upon you but if you catch the Imam in the ruku (bowing), then fatiha is not obligatory upon you.”
[Taken from the lessons on “Conditions, Pillars and Obligation of Salah – Shaykh ul Islaam Muhammad bin Abdil Wahhab رحمه الله Explanation by Shaykh Abdul Muhsin al-Abbaad حفظه الله Taught by: Abdulilah Lahmami]