what should a person do if he joins the prayer when the imaam has alredy finished one rakat and at the end the person forgets that he had missed one rakat and finishes the salah with the imaam, does he have to repeat the whole salah or not?
All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
If a person misses one Rak’ah with the congregational prayer and he joins the prayer with the congregation at the second Rak’ah and he concludes the prayer along with the Imaam out of forgetfulness, and then he remembers, he is obliged to immediately stand up and make up for the Rak’ah that he missed. Then, he should prostrate for forgetfulness after concluding the prayer.
In case he did not stand up to make up for that Rak’ah –out of forgetfulness or out of ignorance –and it was a long time before he remembered, then he is obliged to make up the whole prayer from the beginning.
Allah Knows best.
Fatwa answered by:The Fatwa Center at Islamweb