Saying the salaam at the end of the prayer is obligatory. ‘Ali related that the Prophet said, “The key to prayer is purity. One enters into its inviolable state by the takbir and leaves it by the salaam.”
As to its authenticity, the report is related by Ahmad, ash-Shaf i, Abu Dawud, Ibn Majah and at-Tirmizhi who said, “That is the most authentic report on this topic and the best.”
‘Amr ibn Sa’d related that his father said, “I saw the Prophet making the salaam on his right side and on his left side until I could see the whiteness of his cheeks.” (Related by Ahmad, Muslim, anNasa’i and Ibn Majah.)
Reported Wa’il ibn Hajr, “I prayed with the Messenger of Allah. He would make the salaam on his right side by saying, ‘Peace be upon you and the mercy of Allah.” In Bulugh al-Maram, Ibn Hajr says that Abu Dawud related it with a sahih chain.
It is obligatory to say one salaam, and it is preferred to say two. Ibn al-Munzhir comments that all scholars agree that making only one salaam is permissible. Ibn Qudamah writes in al-Mughni, “There is no clear text from Ahmad that states that two salaams are obligatory. He only said, ‘Two salaams are the most authentic act from the Messenger of Allah.’ It is permissible to say that this is the regulation, although it is not obligatory, and others have the same opinion. This is also pointed out in another of his statements where he said, ‘Two salaams are more loved by me. But ‘Aishah, Salamah ibn al-Aku’ and Sahl ibn Sa’d narrated that the Prophet, upon whom be peace, made only one salaam.” We can reconciliate these differences by stating that it is sunnah to say two salaams, but it is obligatory to say one. This is the consensus that Ibn al-Munzhir mentioned, and we have no option to reject that. Says an-Nawawi, “It is the opinion of ash-Shaifi and most of the early and later scholars that it is sunnah to say two salaams.” Malik and a group of scholars say that only one salaam is sunnah. They adduce this from a weak hadith that can not be used as a proof. If something of this nature had been confirmed from the Prophet, the act was probably done just to show that it is permissible to say only one salaam. Scholars are agreed that only one salaam is obligatory. If one makes only one salaam, he should turn to his right for the first one and to the left for the second one. He should turn until his cheeks can be seen from behind. That is the most authentic form and it is said, “If one says the two salaams to the right or to the left while facing forward, or the first one on the left and the second one on the right, then his prayer would still be valid and he would have fulfilled the act of the two salaams. But, he would have lost the virtue of how they are to be performed.”