He prayed an extra Rak'ah for missing middle Tashahhud
28-3-2013 | IslamWeb
Question:
I was made to lead a magrib prayer and forgot to do tashahud at the end of second rakh. One of the followers said "subhanallah" while I was already on standing for the third rakah.I could not come down to do it. At the end of third rakah, I did tashahud & afterwhich stood for another rakah. But at the end of it all I did sujud bahdi. Is this prayer valid & more explanation on sujud sahwi.
Answer:
All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
You have made a mistake when you offered an extra fourth Rak‘ah in Maghrib prayer instead of the Rak‘ah in which you forgot to recite Tashahhud. It was sufficient for you to offer Sujood As-Sahw (Prostration of forgetfulness) before reciting Salaam. That is because the middle Tashahhud is an obligatory act which can be compensated for by doing Sujood As-Sahw without offering an additional Rak‘ah instead of it.
The basic ruling of the one who offers additional bowing or prostration deliberately is that his prayer becomes invalid. It was stated in Kashshaaf Al-Qinaa‘: "If the worshipper deliberately did some additional action in the prayer, such as standing, sitting, bowing or prostrating, then the prayer is invalidated according to scholarly consensus, because he has violated the order of the prayer and changed its form. Thus, it cannot be considered as a prayer nor can its doer be considered as offering prayer." [End quote]
However, scholars made exception for the one who is not aware of the ruling and they deemed his prayer and the prayer of those who followed him ignorantly valid. It was stated in the Fataawa of the Permanent Committee in Saudi Arabia regarding the Imaam (the one who leads congregational prayer) who offered an additional fifth Rak‘ah ignorantly while he is certain that it is the fifth one: "If the Imaam is not aware of this ruling, then his prayer is valid, Allaah willing. Also, the prayer of those who followed him while they are not aware of the ruling is valid."
Shaykh Ibn 'Uthaymeen 
said about the one who offered an extra Rak‘ah in prayer: "If he is not aware of the ruling, then there is not required to do anything and his prayer is valid. However, if he did that intentionally, then his prayer is invalid because he added an additional Rak‘ah to it."
As for those who followed the Imaam and made an additional Rak‘ah, he (Shaykh Ibn 'Uthaymeen, ) said: "The prayer of those who followed him is also valid if they did that forgetfully or due to ignorance."
As for the rulings of Sujood As-Sahw in details, you can refer to the Fataawa 86560 and 83932. Also, refer to the Fatwa 125209 about the detailed ruling of sitting down for the middle Tashahhud if the person stands up fully for the third Rak‘ah.
Allaah Knows best.