Ruling on writing (S) or (SAWS) etc

What is prescribed is
to write “salla Allaahu ‘alayhi wa salaam (May Allaah send
blessings and peace upon him)” in full, and not to write it in abbreviated
form, such as writing (S) or (SAWS) etc. 

Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allaah have mercy on him)
said: 

As it is prescribed to
send blessings upon the Prophet (peace and blessings of Allaah be upon him)
in prayer when saying the tashahhud, and it is prescribed when giving
khutbahs, saying du’aa’ and praying for forgiveness, and after the adhaan,
and when entering and exiting the mosque, and when mentioning him in other
circumstances, so it is more important to do so when writing his name in a
book, letter, article and so on. So it is prescribed to write the blessing
in full so as to fulfil the command that Allaah has given to us, and so that
the reader will remember to say the blessing when he reads it. So we should
not write the blessing on the Prophet (peace and blessings of Allaah be upon
him) in short form such as writing (S) or (SAWS) etc, or other forms that
some writers use, because that is going against the command of Allaah in His
Book, where He says (interpretation of the meaning): 

“Send your Salaah on
(ask Allaah to bless) him (Muhammad), and (you should) greet (salute) him
with the Islamic way of greeting (salutation, i.e. As‑Salaamu ‘Alaykum)”

[al-Ahzaab 33:56]

And that (writing it in abbreviated form) does not serve
that purpose and is devoid of the virtue of writing “salla Allaahu
‘alayhi wa salaam (May Allaah send blessings and peace upon him)” in
full. Moreover the reader may not take notice of it and may not understand
what is meant by it. It should also be noted that the symbol used for it is
regarded as disapproved by the scholars, who warned against it.