Wa `alaykum As-Salamu waRahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
All scholars are unanimous that sending prayers to the prophet is obligatory once in a Muslim’s life time.
Answering your question, Dr. Mohammad S. Alrahawan, Associate Professor at the department of Islamic Studies in English, Al-Azhar University, Egypt, states:
All scholars are unanimous that sending prayers to the prophet is obligatory once in a Muslim’s life time. They based their view on Allah’s saying:
“Indeed, Allah confers blessing upon the Prophet, and His angels [ask Him to do so]. O you who have believed, ask [Allah to confer] blessing upon him and ask [Allah to grant him] peace.” (Al-Ahzab 33:56)
Imam Ash-Shafi`ī and some Maliki scholars stated that it is obligatory to send prayers to the Prophet at least during obligatory and supererogatory prayers. It is, however, recommended to send prayers on the Prophet when you hear his name at least once.
When writing his name, it is similarly recommended to fully write the verbal formula or its translation. Scholars disliked a person to just put an abbreviation in the form of initial letters of the salla Allahu `alyihi wa sallam.
Allah Almighty knows best.