Blessings and Peace Upon Our Prophet
اللَّهُمَّ صَلِّ وَسَلِّمْ عَلَىٰ نَبِيِّنَا مُحَمَّدٍ
Allahumma salli wa sallim 'ala nabiyyina Muhammad.
O Allah, send blessings and peace upon our Prophet Muhammad.
What it means
A widely used wording that joins the two things Allah commands in the Quran, blessings and peace, in one short sentence. The instruction to send blessings after the call to prayer is established in the sunnah, while this particular phrasing is one of the accepted forms scholars and teachers have long used.
When and how
Say it after answering the muadhdhin and before asking for al-wasilah for the Prophet ﷺ.
Its virtue
The Prophet ﷺ said that when you hear the muadhdhin, repeat what he says, then send blessings upon him, for whoever sends blessings upon him once, Allah sends blessings upon him ten times, reported in Sahih Muslim 384.
Source
Muslim 384 (the instruction to send blessings after the adhan)