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.
Whoever hears the mention of the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, is required to confer blessings upon him. That is because the Prophet, sallallaahu ‘alayhi wa sallam, said: “The stingy person is he who does not send blessings on me whenever I am mentioned before him.” [At-Tirmithi]
As for the sentence of 'sallallaahu ‘alayhi wa sallam (may Allaah send blessing and speace upon him)', it was common during the time of the Companions .
The word 'Sallam' is pronounced with doubling the letter Laam (i.e. having the Shaddah [germination mark]). However, there is no doubt that the best wording of sending peace and blessings upon the Prophet is As-Salah Al-Ibraaheemiyyah (i.e. the wording said at the final sitting in the prayer) as this was stated in the authentic Hadeeth. For more benefit, kindly refer to Fatwa 92222.
Allaah Knows best.