وَالصَّلَاةُ عَلَى نَبِيِّهِ مُحَمَّدٍ
And blessings upon His Prophet Muhammad, peace and blessings be upon him.
Among the basic lexical meanings of ṣalāt is supplication. In this context scholars distinguish the believer's prayer for the Prophet from Allah's bestowal of mercy, honor and exalted mention as befits divine action. God's ṣalāt is not a needy request like a human prayer.
Mentioning the Messenger after praising Allah connects recognition of the true object of worship with acknowledgment of the messenger through whom revealed guidance reaches us.
Distinction: The Arabic term ṣalāt can also denote the ritual prayer, but not every occurrence has that technical meaning; context and grammatical construction determine its use.