Answered by Shaykh Amjad Rasheed
What should I do if the imam finishes his prayer by saying “salamu alaykum wa rahmatullah” without either the definite article [h: “al-salamu alaykum ”] or the nunation [h: “salamun alaykum ”]? [h: The reason for this question is that in the Shafii school, the final salams must be said with the definite article (“al”), i.e., “al-salamu ” Merely saying “salamu ” is not sufficient and results in an invalid prayer. See Reliance of the Traveller, f8.47]
The follower should simply say his salams after the imam: [h: however the imam says his salams, it does not affect the validity of the followers prayer] and the follower does not have to do anything.
The reason for this is that [h: the imam did one of three things]: (1) he either said the definite article but the follower did not hear itin this case there is no harm doneor (2) he deliberately said the salams without the definite article knowing full well that it was unlawful to do so, thereby invalidating his prayerin this case the follower becomes as if he is praying alone (munfarid) when the imams prayer is invalidated [h: and he simply completes his own prayer by saying the salams]or (3) the imam was excused in his doing so [h: by doing it out of ignorance, for example]in which case it is valid to follow him as scholars have explicitly stated. [h: This last case] is different from [h: an imam] who makes mistakes in his fatiha [h: where it would not be valid to follow him].
(Translated by Hamza Karamali)