Is it permissible to make Tasleem only once at the end of the prayer by saying "As-Salaamu ‘alaykum wa rahmatullaah" only once? May Allaah Reward you.

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 should know that the first Tasleem is obligatory while the second one is recommended. Consequently, the prayer is valid when one makes Tasleem only once. In Al-Majmoo‘, An-Nawawi said, “Ibn Al-Munthir said, ‘The scholars unanimously agree that it is permissible to make one Tasleem only.’ When one decides to make one Tasleem only, it is recommended that he looks in front of him; otherwise, he should turn to his right during the first and then to his left during the second. With each time making Tasleem, he should turn until the one on his side sees his cheek.”