Prayer: Becoming Aware of the Obligation to Perform Tamam During Prayer
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1351
If a traveller who must perform tamām prayers on account of not knowing the ruling starts performing prayers with the intention of performing a two rakʿah prayer, and he becomes aware of the ruling during prayers, he must complete the prayer as a four rakʿah prayer. And the recommended precaution is that after completing the prayer, he should perform that prayer again as a four rakʿah prayer.