The Short Salawat
اللَّهُمَّ صَلِّ عَلَىٰ مُحَمَّدٍ
Allahumma salli 'ala Muhammad.
O Allah, send blessings upon Muhammad.
What it means
The briefest complete form, and the one to use when you want to send blessings many times over. The Prophet ﷺ instructed that after answering the call to prayer we send blessings upon him, and then ask Allah for al-wasilah for him.
When and how
Say it after the adhan, when you hear his name, and as a light dhikr to repeat while walking or waiting.
Its virtue
The Prophet ﷺ said that whoever sends blessings upon him once, Allah sends blessings upon him ten times in return, reported in Sahih Muslim 384.
Source
Muslim 384sahih