Prayer: Prolonging Rukuʿ When a Person Arrives to Join
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1466
If while performing rukūʿ the imam realises that a person has just arrived and wants to join, it is recommended that he prolong the rukūʿ twice as much as normal and then stand up, even if he realises that another person has also arrived and wants to join.