Saying ‘Assalaamu Alaykum’ in Tasleem
29-10-2016 | IslamWeb
Question:
Assalaamu 'alaykum. If the imam of the mosque, following Tasleem, says, "Salam 'alaykum wa rahmatullah" or "Assalam 'alaykum wa rahmatullah" or "Salamu 'alaykum wa rahmatullah," is the prayer valid? Can you pray behind such an imam? May Allaah reward you.
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.
Tasleem is one of the pillars of prayer, and it must be pronounced with the definite article as follows, “As-salaamu ‘Alaykum.” Saying “Salaam ‘Alaykum” is insufficient, and so is “Salaamoo ‘Alaykum” because of the addition of the letter “waw” at the end of the word "Salaam;" which has no basis in the Arabic language.
An-Nawawi wrote:
“As for the ruling on the Tasleem, it is one of the pillars of the prayer without which the prayer is invalid, and no other act can replace it. The least sufficient version is to say ‘As-Salaamu ‘Alaykum.’ If he mispronounces one of these letters, then his Tasleem is not valid. If he, for instance, said, ‘As-Salaamu ‘Alayk, Salaamullaahi ‘Alaykum, Salaamun ‘Alaykum or As-Salaamu ‘Alayhim,’ it is not sufficient and his Tasleem is invalid. If he mispronounces the Tasleem out of forgetfulness, then his prayer is not invalidated and he should perform Sujood as-Sahw (prostration of forgetfulness) and he has to repeat the Tasleem in this case. If he mispronounces the Tasleem deliberately, then his prayer is invalid...” [Al-Majmoo’]
The prayer is invalidated by mispronouncing the Tasleem deliberately while knowing of the prohibition of mispronouncing it, contrary to the case of forgetfulness or ignorance of the prohibition in this regard. Al-Khateeb Ash-Shirbeeni 
wrote, “It is not sufficient to say, As-Salaamu ‘Alayhim, As-Salaamu ‘Alayka, As-Salaamu ‘Alaykuma, Salaami ‘Alaykum, or Salaamullaahi ‘Alaykum. If the praying person mispronounces the Tasleem deliberately while knowing of the prohibition of such an act, then his prayer is invalid.” [Mughni Al-Muhtaaj]
Al-Bujayrimi in his commentary on Al-Khateeb said, “If the praying person says while reciting Tasleem, ‘Salaamu ‘Alaykum,’ then his prayer is invalidated. This is the most likely correct view, although some scholars held otherwise. However, the ruling in this regard is restricted to the absence of the factor of ignorance which entails excusing the doer...”
Pronouncing the letter “m” with Sukoon (instead the short vowel Dhammah, i.e. saying As-Salaam ‘Alaykum instead of As-Salaamu ‘Alaykum) does not invalidate the prayer as long as the definite article “As” is pronounced. Al-Bujayrimi also wrote, “The least acceptable version of Tasleem in the prayer is saying As-Salaamu ‘Alaykum, even if pronounced As-Salaam ‘Alaykum, pronouncing the “m” with Sukoon.”
In brief, reciting the Tasleem without the definite article, saying Salaamu ‘Alaykum instead of As-Salaamu ‘Alaykum, is not sufficient. However, it does not invalidate the prayer if it is done out of ignorance of the relevant ruling or out of forgetfulness or as a mistake.
Hence, you come to know the religious ruling on performing prayers behind the mentioned imam.
Allaah knows best.