بسم الله الرحمن الرحيم
The following are the excerpts from Fatwas of Ibn Baz – read the full article at http://www.alifta.net
This was published in the magazine of the Islamic University in Madinah, the fourth edition, the sixth year, Rabi` Al-Thany, 1394 A.H., pp. 175-182.
All Praise is due to Allah. May peace be upon the Last Prophet, his family, and his Companions.
Imam Muslim related in his Sahih (Book of sound Hadiths) from Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace be upon him) said:
“If anyone invokes blessings on me once, Allah will bless him ten times” 
It is also narrated that the Messenger of Allah (peace be upon him) said:
“Do not make your houses graves and do not make my grave a place of celebration but invoke blessings on me, for your blessings reach me wherever you may be” 
He (May peace and blessings be upon him) also said:
“May his nose soil with dust in whose presence mention is made of me and he does not supplicate for me” 
Since invoking peace and blessings on the Prophet (peace be upon him) is permissible in Salah during Tashahhud (a recitation in the sitting position in the second/last unit of Prayer) and permissible in Khutbah (sermon), invocations, seeking forgiveness, after Adhan (Call to Prayer), upon entering, and leaving the Masjid (mosque), when uttering his name and in other positions.
It is confirmed to write the form of invoking peace and blessings when writing His name in a book, a message, or an article, because of the proofs mentioned above. It is permissible to write the full form in order to apply what Allah has commanded and the reader would remember it when passing by it.
You should not reduce it to the acronym e.g. (P) that stands for peace or (PBUH) that stands for (peace be upon him) or the like abbreviations that may be used by some writers.Such an act is against the ordinance of Allah (May He be Praised and Glorified) which are mentioned in His Book by His saying: Send your Salât on (ask Allâh to bless) him (Muhammad صلى الله عليه وسلم), and (you should) greet (salute) him with the Islâmic way of greeting (salutation i.e. As-Salâmu ‘Alaikum).[Surah AlAhzab, 33: 56] It does not have the same privilege of writing (peace be upon him) in full form. The reader may not watch out or may not understand the meaning, bearing in mind that referring to it is disapproved by the religious scholars who even warned against it.
Ibn Al-Salah said in his book: “`Ulum al-Hadith” i.e. The sciences of Hadith, known as “Muqadimat Ibn Al-Salah” in the twenty fifth type of his book: (Al-Hadith wa Kayfiyat Dabt-ul-kitab wa Taqyiduh). He said that which means:
Ninth: A person must keep writing the form of sending Salat and taslim i.e. “peace and blessing be upon the Messenger of Allah” in full form upon mentioning him. A person must not be weary of repeating it, because it is one of the greatest benefits which the Hadith students and writers may overlook. Whoever disregards this, he will be deprived of a great reward. We have seen good visions for the people who write the form in full, because it is a form that a person writes as a Du`a’ (supplication), not a part of the narration to make it dependent on the source of narration.
Similarly, the same is true with regard to praising Allah (Glorified be He) upon mentioning His Name such as (Glorified and Exalted be He), (Blessed and Exalted be He), and the like… (Until he says): then a person should avoid two shortcomings:
- First: Writing it in incomplete form by two letters or the like.
- Second: Writing it in short form by meaning such as (wa sallam i.e. blessings).
It is narrated from Hamzah Al-Kinany (may Allah be merciful to him) that he used to say:
I used to record the Hadith and I used to write down upon mentioning the name of the Prophet (salla-Allah-u-`alaihi i.e. May Allah’s peace be upon him) without writing (wa sallam). Thereupon, I saw the Prophet (peace be upon him) in a vision saying to me: Why do you not complete the form of invoking peace and blessingson me? He continued: Ever since, I did not write it in short form like this (salla-Allah-u-`alaihi) but in this form (salla-Allah-u-`alaihi-wa-sallam i.e. May Allah’s peace and blessings be upon him). Ibn Al-Salah added: I said: it is also reprehensible to write (`alaihi-s-salam i.e. peace upon him) and Allah knows best.
End of quote.
His authority, the encyclopedic scholar Al-Sakhawy (May Allah be merciful with him) said in his book (Fath-ul-Mughith Sharh Alfiyat Al-Hadith by Al-`Iraqy) :
O writer, avoid (referring to it) i.e. peace and blessings upon the Messenger of Allah (peace be upon him) by writing only two letters or the like as done by Al-Kattany and the ignorant of the non-Arabs and ignorant students. They write instead of (May Allah’s peace and blessings be upon him), (p) or (pb) or (pbuh) and that entails decrease in reward, because it is unlike the proper manner.
Al-Suyuty (May Allah be merciful with him) said in his book (Tadrib Al-Rawy Fi Sharh Taqrib Al-Nawawy):
It is reprehensible to write the short form of peace and blessings here and in every position where invoking peace and blessings is required as mentioned in the commentary of Sahih Muslim and others because of Allah’s saying: Send your Salât on (ask Allâh to bless) him (Muhammad صلى الله عليه وسلم), and (you should) greet (salute) him with the Islâmic way of greeting (salutation i.e. As-Salâmu ‘Alaikum).[Surah AlAhzab, 33: 56] Until he says: It is reprehensible to refer to it with one or two letters as those people who write (pbuh), he must write it in full form.
End of quote, adapted.
My advice for every Muslim; reader or writer, is to seek for the best reward and stay away from what may invalidate or reduce it.
We ask Allah to grant us success to that which pleases Him, for He is the Most Generous. May peace and blessings be upon our Prophet Muhammad, his family, and Companions!
 Muslim, Sahih, Book on Salah, no. 408; Al-Tirmidhy, Sunan, Book on Salah, no. 485; Al-Nasa’y, Sunan, Book on Sujud-ul-Sahw, no. 1296; Abu Dawud, Sunan, Book on Salah, no. 1530; and Ahmad ibn Hanbal, Musnad, vol. 2, p. 372.
 Related by Abu Dawud, Book on rituals, Chapter on visiting graveyards; and Ahmad, vol. 2, p. 316.
 Related by Al-Tirmidhy, Book on supplications, ranked as Hasan Gharib.