"Surely,Allah and his angles send the blessings to the prophet, O those who believe, do pray Allah to bless him, and send your Salam to him in abundance." 33:56
Ibn Abbas said the meaning of this is that Allah and his angles invoke blessings on the Prophet. It is said that Allah showers mercy on the Prophet and his angles supplicate to him.
Al-Mubarrad said,"The root of the prayer is invoking mercy, It is mercy from Allah, and from the angles it is graciousness and supplication for Allah's mercy."
from ash-Shifa of Qadi 'Iyad Chapter 4 "The Prayer on the Prophet and asking Peace for him and the obligation of doing it and its excellence"