Allah's Command to send salutation upon the Holy Prophet sallallahu alayhi wasallam

We, with all our shortcomings, are unable to repay the immense favours of such a great benefactor. In view of our helplessness, Allah, the Most Gracious, ordained a great but very easy act in an extraordinary manner. By this action we can at least nominally express our gratitude to His Messenger sallallahu alayhi wasallam.
Allah,the Most Merciful has revealed:
"Verily, Allah and His angels shower blessings on the Prophet. O Believers! send blessings upon him and salute him with a worthy salutation". (Qur'aan - 33:56)

Commantary
Allah has revealed many commandments in the Holy Qur’aan such as Prayers, Fasting, Hajj, Zakaat etc., and has related the noble qualities of many Prophets. However, in none of these injunctions about acts of worship or in neither of the praise about the Prophets, has He ever said that we should honour someone because He honors him too.
The leader of the Apostles, our Holy Prophet sallallahu alayhi wasallam is the only one to be ever crowned with this supreme honour, in that Allah has first related an action unto Himself, then with His angels, and finally commanded all the believers to send blessings to the Holy Prophet sallallahu alayhi wasallam. It is a great Favour that the believers are permitted to send blessings as in so doing they participate in an act of Allah and His angels.

From this verse, the perfection accorded to the Holy Prophet sallallahu alayhi wasallam became known to all, as Allah also sends blessings on him. This is also the reason of excellence of this action and it is from amongst the distinctions of being Allah's Beloved.

The importance of sending blessings (Ahaadeeth of the Holy Prophet sallallahu alayhi wasallam)