Peace Be Upon You, O Prophet
السَّلَامُ عَلَيْكَ أَيُّهَا النَّبِيُّ وَرَحْمَةُ اللَّهِ وَبَرَكَاتُهُ
As-salamu 'alayka ayyuhan-nabiyyu wa rahmatullahi wa barakatuh.
Peace be upon you, O Prophet, and the mercy of Allah and His blessings.
What it means
Allah commanded the believers to send both blessings and greetings of peace upon His Messenger ﷺ. This line, part of the tashahhud taught by the Prophet ﷺ to Ibn Masud, is how that greeting is said, and every Muslim repeats it in every prayer of their life.
When and how
Say it as part of the tashahhud in every prayer, and greet him with peace whenever you send blessings upon him.
Its virtue
Part of the tashahhud that the Prophet ﷺ taught word by word, as Ibn Masud described, reported in Sahih al-Bukhari 831. The Prophet ﷺ also said that Allah has appointed angels who convey to him the greetings of his ummah, reported in Sunan an-Nasai.
Source
Al-Bukhari 831sahih