Assalaamu alaykum wa rahmatullaahi wa barakaatuhu. I have a question regarding the prayer: If I forgot to say only the sentence of the Shahadah (testimony of faith - Ash-hadu an-la ilaha illa allah wa ash-hadu anna Muhammadan abduhu wa rasuluh) in the first and/or second Tashahhud (I said the whole Tashahhud but I forgot to say only this sentence), then should I perform Sujood Sahw (prostration of forgetfulness) before making Tasleem (ending the prayer with the Salam), or is my Tashahhud still correct? What if this happened to me while praying behind the imam in the congregational prayer and I remembered after finishing the second unit of the prayer that I forgot to say the sentence of the Tashahhud or after finishing prayer that I forgot to say the sentence of the Tashahhud in the second Tashahhud; what should I do then?

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, sallallahu ‘alayhi wa sallam, is His slave and Messenger. 

According to the majority of the scholars, the first Tashahhud in the prayer is one of the Sunnah acts of the prayer, but the Hanbalis held that it is among the obligations of the prayer and that it must be recited in full. The Hanbali book Daqaa'iq Uli An-Nuha Sharh Al-Muntaha reads, “The shortest sufficient version of the first Tashahhud is, 'At-Tahiyyaatu lillah. Salaamun ‘alayka ayyuhan-Nabiyyu wa Rahmatullah. Salaamun ‘alayna wa ‘ala ‘ibaadillahi As-Saaliheen; Ash-hadu an-la ilaaha illa Allah wa ash-hadu anna Muhammadan Rasoolullah,' or, 'Muhammadan ‘Abduhu wa Rasooluh.' Whoever misses a letter of the Tashahhud intentionally, his prayer is invalid given that this version is the agreed upon one in all the relevant ahaadeeth.”

If you forgot a sentence of the first Tashahhud while praying individually, you should perform Sujood as-Sahw before Tasleem. The Hanbali book Kashshaaf Al-Qinaa’ 'An Matn Al-Iqnaa’, pertaining to the obligations of the prayer, reads, “The second category of the acts of the prayer consists of the obligations. When the worshiper misses any of the obligations of the prayer deliberately, his prayer is invalidated. However, if he missed it out of forgetfulness or ignorance, then the prayer is not invalidated, and it is compensated for by performing Sujood as-Sahw, as reported from Imaam Ahmad. This does not apply to the conditions and pillars of the prayer....”

If you are praying behind an imaam in the congregational prayer, then you are not required to perform Sujood as-Sahw because the imaam bears it on your behalf. The Hanbali book Daqaa-'iq Uli An-Nuha Ma’a sharh Muntaha Al-Iraadaat reads, “If the one being led in the prayer forgets an act of the prayer while the imaam did not forget it, then he is not required to perform Sujood as-Sahw unless the imaam himself forgot something and made Sujood as-Sahw. In this case, the one being led in the prayer should follow his imaam in performing Sujood as-Sahw.”

This ruling applies to the person who misses a part of the first Tashahhud. However, the ruling on the final Tashahhud is different because, according to the preponderant view of the Shaafi‘is and Hanbalis, it is among the pillars of the prayer, and that is the view adopted at Islamweb. Hence, if you forgot a sentence of the final Tashahhud and did not remember that except after Tasleem, then you should recite it regardless of whether you were performing the prayer individually or in congregation provided that there is no long interval of time between the Tasleem and reciting what you missed. The criterion in estimating the long interval of time in this context is what is customary and usual to the people. In case of a long interval of time between the Tasleem and reciting the missed part of Tashahhud, the prayer is invalid.

The Hanbali scholar Al-Mardaawi while addressing the case of missing one of the pillars of the prayer, wrote, “The preponderant view according to the Hanbalis is that if the person does not know that he missed one of the pillars of the prayer except after making Tasleem, his prayer is valid, and it is like missing a Rak'ah of the prayer, meaning that he makes up what he missed, but only if there was a short interval of time between the Tasleem and performing the missed pillar of the prayer, even if he moved away from the direction of the Qiblah or left the mosque.” [Al-Insaaf]

Allah knows best.