Prayer: Joining from the Beginning or During Recitation
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1410
If a person joins a congregational prayer from the beginning or during Sūrat al-Ḥamd and the other surah, and if it so happens that before he goes to rukūʿ the imam raises his head from rukūʿ, his congregational prayer is valid.