Prayer: Being One Rakʿah Behind the Imam
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1432
With regard to someone who is one rakʿah behind the imam, it is better that when the imam says tashahhud of the last rakʿah, he should sit [in a squatted position] such that his fingers and the balls of his feet are on the ground and his knees are off the ground, and he should wait until the imam has said the salām of the prayer and then stand up. If at that point he wants to make an intention of performing the prayer on his own, there is no problem.