Prayer: Completing a Three or Four Rakʿah Prayer as a Recommended Prayer to Join Congregational Prayers
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1430
If while a person is performing a three rakʿah or four rakʿah prayer a congregational prayer commences, in the event that the congregational prayer is for the same prayer that the person is performing, and he has not gone into the rukūʿ of the third rakʿah, and he is not confident that if he completes the prayer he will be able to join the congregational prayer, it is recommended that he complete the prayer as a two rakʿah prayer with the intention of a recommended prayer and join the congregational prayer.