Reference: Fataawa al-Lajnah ad-Daa.imah lil-Buhooth al-‘Ilmiyyah wal-Iftaa., – Volume 12, Page 208, Question 3 of Fatwa No.5069
Question: Is it permissible to write the letter (S) [ص] instead of [writing] «sal-Allaahu ‘alayhi wa sallam», and why?
Response: The Sunnah is to write the entire phrase «sal-Allaahu ‘alayhi wa sallam» – since it is a supplication; And the supplication is a [form of] worship, such as verbally saying it (the phrase), and the abbreviation for it using the letter (S) [ص] or (SAW) [ص ع و] [or (SAAW) or the like] is not [constitute] a supplication, and nor a [form of] worship, whether it be by speech or writing. And because of this, this abbreviation was not used by those of the first three generations about whom the Prophet ﷺ bore witness to their excellence.
And with Allaah lies all success and may Allaah send prayers and salutations upon our Prophet Muhammad ﷺ and his family and his companions.