The Baraqah is in the word

"Allah and His angels send blessings on the Prophet: O ye that believe! Send ye blessings on him, and salute him with all respect." (33,56)

Question- Is it permissible to write the letter (S) instead of written (Salla Allahu alayhi wa sallam) , and why?

Answer- The sunnah is to write the entire phrase (Salla Allahu 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) or Salla Allahu alayhi wa sallam , (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 (Salla Allahu alayhi wa sallam) bore witness to their excellence.

And with Allah lies all sucess and may Allah sends prayers and salutations upon our Prophet (Salla Allahu alayhi wa sallam) and his family and his companions.

The Permanent Committee for Islaamic Research and Fataawa, comprising -

Head: Shaykh 'Abdul 'Azeez Ibn Abdullaah Ibn Baaz;
Member: Shaykh 'Abdur-Razzaaq 'Afeefee;
Member: Shaykh 'Abdullaah Ibn Ghudayyaan;
Member: Shaykh 'Abdullaah Ibn Qu'ood
 SOURCE:Fataawa al-Lajnah ad-Daa.imah lil-Buhooth al-'Ilmiyyah wal-Iftaa., - Volume 12, Page 208, Question 3 of Fatwa No.5069