When His Name Is Mentioned
صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ
Sallallahu 'alayhi wa sallam.
May Allah send blessings and peace upon him.
What it means
This is the familiar phrase said and written after the Prophet's name. It is a supplication in the form of a statement, asking Allah to bless and grant peace to him, and it is the established practice of the Muslims whenever he is mentioned.
When and how
Say it aloud whenever his name is mentioned in speech, lectures or reading, and write it in full rather than as an abbreviation where you can.
Its virtue
The Prophet ﷺ said that a person is humiliated in whose presence he is mentioned and who then fails to send blessings upon him, reported in Jami at-Tirmidhi 3545.
Source
At-Tirmidhi 3545 (the warning against neglecting it)